I'm not aware of any such cases and I don't think that there are any, because if there was, the case would have generated more publicity around a "glamorous" issue in the law.
Despite the fact that constitutional law makes it much easier to bring defamation cases when one is a private individual suing a non-media defendant in a matter of private concern, any search of the case law reveals that the defamation cases that actually get brought are those against media defendants and those involving matters of public concern.
Private individuals rarely suffer enough harm to make it economic to bring a defamation suit, and people whose lives are not a matter of public concern rarely have deep pockets to hire attorneys to bring defamation lawsuits.
Only Nominal Damages And No Attorneys' Fees Could Be Recovered
First of all, even in a negligence per se case, where an award of nominal damages (i.e. $1 and court costs excluding attorneys' fees) is allowed as matter of law to the prevailing party, I don't think that an award of more than nominal damages would be upheld in the face of affirmative evidence that there was no actual harm to the reputation of the person defamed as a result of the publication of the defamatory statement.
In defamation per se cases, nominal damages are awarded when "there is no proof that serious harm has resulted from the defendant's attack upon the plaintiff's character and reputation" or "when they are the only damages claimed, and the action is brought for the purpose of vindicating the plaintiff's character by a verdict of a jury that establishes the falsity of the defamatory matter." RESTATEMENT (SECOND) OF TORTS § 620 cmt. a (1977). This would apply in a case of complete disbelief.
Since the American rule applies to attorneys' fees in defamation cases, this would make every such suit an economic loser - which doesn't mean that someone might not act in an economically irrational way to defend their honor or something like that.
Suing Increases The Harm Rather Than Mitigating It
The publicity of a public trial undermines that approach in the kind of case in the hypothetical as well. Absent a lawsuit, there are one or two people who don't even believe it who heard the defamatory statement.
But, if you bring a lawsuit, given the likelihood that the media will cover such a case, millions of people will hear the defamatory statement and they may very well believe those statements because they don't know any better. Even if you are ultimately vindicated at the conclusion of a trial, many people will have heard the defamatory statements after the suit is filed, but will never find out that you were vindicated many months later following a trial.
Massively spreading defamatory statements about yourself that nobody would otherwise have heard about is just stupid as a matter of litigation tactics.
The Presumption Of Harm To Reputation May Be Rebuttable
Secondly, it isn't obvious to me that the presumption of harm to reputation in a negligence per se case in a conclusive presumption as opposed to a rebuttable presumption. For example, one can generally argue in a defamation case that someone's reputation before the defamatory statement was made is so irretrievably bad in the area related to the defamatory statement that it is impossible to damage someone's reputation any further, and so far as I know, that argument is not prohibited in negligence per se cases.
For example, an intentionally false defamatory statement that Ted Bundy once punched a prostitute in the nose giving her a black eye at the Moonbeam Bar at a particular date in the midst of Ted Bundy's serial killing spree (which he can prove is false with an iron clad alibi and which the maker of the statement admits was made up at trial), while constituting negligence per se might not state a claim for relief given that Ted Bundy's reputation for not being a violent criminal is already hopeless tarnished by his multiple murder convictions for similar conduct.
Milkovic Can Be Evaluated In Context
Third, I am inclined to think that Milkovich v. Lorain Journal, 497 U.S. 1, one of the holdings is that "statements that cannot reasonably be interpreted as stating actual facts about an individual are protected", together with cases such as the New York Times case reflect an evolving understanding in the law (quite distinct from the British concept of defamation which has a bit of an "if you don't have something nice to say don't say anything at all" to it), that defamation is simply a special kind of fraud claim in U.S. law, and that defamation is only constitutional in the United States because it is a form of fraud claim.
If defamation is and must be merely a special kind of fraud, then the reliance element of a fraud claim is probably constitutionally required in a case where the existence or absence of reliance can be discerned as a matter of fact, without resorting to generalities as court in Milkovich had to since the statement was published to a large number of people. It would not be unreasonable (and arguably constitutionally mandatory) to read a gloss on the "statements that cannot reasonably be interpreted as stating actual facts about an individual are protected" standard of Milkovich, to include an implied "by the people to whom the statement was published" clause, which would have one meaning when a large number of people in the general public heard it, and another in a context when only one or two people actually heard the statement, or when it was only heard by a group of people who would interpret it differently than the general public would. And, if so, that would be a complete defense and would not just reduce the claim to one limited to nominal damages.
This is not a great stretch. For example, in California the words of an alleged libel must be considered "according to the sense and meaning under all the circumstances attending the publication which such language may fairly be presumed to have conveyed to those to whom it was published." Macleod v. Tribune Publishing Co., 52 Cal.2d 536, 546-547; Selleck v. Globe International, Inc., 166 Cal.App.3d 1123, 1132.
Libel Per Se No Longer Exists For Media Defendants Absent Actual Malice
At one point it looked like the case Gertz v. Robert Welch, Inc., 481 U.S. 323, 349-350 (1974) might constitutionally eliminate libel per se, but this was premature. Gertz does not apply in cases involving matters of private concern to private individuals where the defendants are not media defendants. Dun & Bradstreet, Inc. v. Greenmoss Builders, Inc., 472 U.S. 749, 761 (1985) (where a defendant's speech concerned a private individual and a matter of private concern, states could allow plaintiffs to recover presumed and punitive damages even absent a showing of actual malice).
But, Dun & Bradstreet didn't address the question of whether the presumption of damages in a libel per se case was a conclusive presumption or a rebuttable one, because if it is a rebuttable presumption, then it could be overcome in the hypothetical of the question. And, it also doesn't address the question of whether the Milkovich analysis in a libel per se case must be context specific.
In a case involving a media defendant and a private individual plaintiff the U.S. Supreme Court held in Gertz v. Robert Welch, Inc., 481 U.S. 323, 349-350 (1974) that:
States may not permit recovery of presumed or punitive damages, at
least when liability is not based on a showing of knowledge of falsity
or reckless disregard for the truth.
The common law of defamation is an oddity of tort law, for it allows
recovery of purportedly compensatory damages without evidence of
actual loss. Under the traditional rules pertaining to actions for
libel, the existence of injury is presumed from the fact of
publication. Juries may award substantial sums as compensation for
supposed damage to reputation without any proof that such harm
actually occurred. The largely uncontrolled discretion of juries to
award damages where there is no loss unnecessarily compounds the
potential of any system of liability for defamatory
falsehood to inhibit the vigorous exercise of First Amendment
freedoms. Additionally, the doctrine of presumed damages invites
juries to punish unpopular opinion, rather than to compensate
individuals for injury sustained by the publication of a false fact.
More to the point, the States have no substantial interest in securing
for plaintiffs such as this petitioner gratuitous awards of money
damages far in excess of any actual injury.
We would not, of course, invalidate state law simply because we doubt
its wisdom, but here we are attempting to reconcile state law with a
competing interest grounded in the constitutional command of the First
Amendment. It is therefore appropriate to require that state remedies
for defamatory falsehood reach no farther than is necessary to protect
the legitimate interest involved. It is necessary to restrict
defamation plaintiffs who do not prove knowledge of falsity or
reckless disregard for the truth to compensation for actual injury. We
need not define "actual injury," as trial courts have wide experience
in framing appropriate jury instructions in tort actions. Suffice it
to say that actual injury is not limited to out-of-pocket loss.
Indeed, the more customary types of actual harm inflicted by
defamatory falsehood include impairment of reputation and standing in
the community, personal humiliation, and mental anguish and suffering.
Of course, juries must be limited by appropriate instructions, and all
awards must be supported by competent evidence concerning the injury,
although there need be no evidence which assigns an actual dollar
value to the injury.
We also find no justification for allowing awards of punitive damages
against publishers and broadcasters held liable under state-defined
standards of liability for defamation. In most jurisdictions jury
discretion over the amounts awarded is limited only by the gentle rule
that they not be excessive. Consequently, juries assess punitive
damages in wholly unpredictable amounts bearing no necessary relation
to the actual harm caused. And they remain free to use their
discretion selectively to punish expressions of unpopular views. Like
the doctrine of presumed damages, jury discretion to award punitive
damages unnecessarily exacerbates the danger of media self-censorship,
but, unlike the former rule, punitive damages are wholly irrelevant to
the state interest that justifies a negligence standard for private
defamation actions. They are not compensation for injury. Instead,
they are private fines levied by civil juries to punish reprehensible
conduct and to deter its future occurrence. In short, the private
defamation plaintiff who establishes liability under a less demanding
standard than that stated by New York Times may recover only such
damages as are sufficient to compensate him for actual injury.
See also Time, Inc. v. Firestone, 424 U.S. 448, 459 (1976).
In the cases where it applies, Gertz requires proof of actual damages and bars the award of punitive damages, as a First Amendment requirement, and since actual damages are entirely absent in the case of the defendants in the question's hypothetical, if Gertz applied to them, they would not prevail. Now, Gertz in 1974 when it was decided, had limitations - it involved media defendants for whom actual malice could not be shown, but it did eliminate the public figure/public concern requirement. But, it isn't obvious to me that the Gertz limitations have not been expanded since then. Libel per se no longer exists in cases governed by Gertz (i.e. media defendants for whom there is no showing of actual malice). From v. Tallahassee Democrat, Inc., 400 So.2d 52 (Fla. App. 1981).
Texas has expanded Gertz somewhat and held that even in cases where it does not apply, actual proof of actual damages is required to recover exemplary damages, even in libel per se cases where damages are presumed. Doubleday & Co., Inc. v. Rogers, 674 S.W.2d 751, 755 (Tex. 1984).