Can one rely on copyright holder's vague language?

Question

The author of the Code2000 font stated:

Users are required to register the font after a “reasonable” evaluation period if they like the font and continue to use it. However, determining what is “reasonable” is left for the user to decide. The Code2000 download doesn’t degrade or expire and there are no annoying pop-up screens. This has been left open-ended intentionally. In some cases, members of minority script user communites — those who need a font like Code2000 the most — can least afford it. Clearly, if registering the font means your family doesn’t get enough food on the table, even for one meal, then it is not reasonable to register the font.

But, if you can afford the small fee, and you like the font and continue to use it after a reasonable evaluation period, then register your shareware! After all, I need food on my plate, too.

Now, since the author has disappeared (the PayPal account specified for payments is part of a domain registration that expired and was later repurposed), one could argue that,

• no amount of time is “reasonable”,
• tracking down the copyright holder would entail enormous costs (having to contact hundreds of people), so that that one's “family doesn’t get enough food on the table”,
• one cannot “afford the small fee” (though it is only USD 5.00),

but could these be enough grounds for continuous free usage without risk?

Answer 1

could these be enough grounds for continuous free usage without risk?

No. Besides abusing the author's good faith, the matter would be actionable under legal theories including --but not limited to-- copyright infringement and breach of contract. I'm assuming the copyright laws in your jurisdiction are or resemble those of the US or the European Union.

no amount of time is “reasonable”

The clause actually provides guidance on how to determine that a delay or lack of registration is reasonable. The criterion mostly is in terms of user's financial capability to make the payment.

If the copyright holder files suit and the defendant alleges he cannot afford the payment, the fact-finder would need to assess whether(when) the defendant's finances enabled him to pay the fee. In the affirmative, the defendant would be liable to the copyright holder.

one cannot “afford the small fee” (though it is only USD 5.00)

That allegation is most likely to be ruled as frivolous unless the defendant lives in a severely undeveloped economy where USD 5.00 entails significant purchase power. And that scenario seems inconsistent with having enough technology to download and use the font at issue.

tracking down the copyright holder would entail enormous costs (having to contact hundreds of people), so that that one's “family doesn’t get enough food on the table”

The fact that required fee is explicitly, and only, USD 5 relieves the user of the burden of ensuring that the author's entire family gets "enough food on the table". The author is just friendly about [needlessly] justifying the fee he requires. But that friendliness ought not to be construed as a loophole, since the contractual intent is very explicit: A fee of 5 USD in exchange for continued use of the font.

Nor is the author's disappearance (?) a valid argument for eluding payment. The author provided a commonplace method (PayPal) for registering the usage of the font.

Even if by disappearance you mean that the author is dead, the copyright holder (or beneficiary) would prevail. See Petrella v. Metro-Goldwin-Myer, Inc., 134 S.Ct. 1962, 1968 (2014) "works are generally protected from the date of creation until 70 years after the author's death" (citing Copyright Act, 17 USC §302(a)).