A high profile example of modern seppuku was Yukio Mishima in 1970. His second failed to behead him and committed seppuku too, with both being beheaded by Hiroyasu Koga.
A newspaper at the time noted that, in addition to being charged with crimes as a revolutionary, Koga was charged with "murder by request" (a.k.a. assisting suicide), along with two others present as accomplices.
The crime against Koga is that of "murder by request" or "assisting suicide." And, according to precedent, those guilty of "murder by request" receive lighter sentences than those convicted of first-degree murder. Will Koga thus receive a lenient sentence because he merely assisted in another's suicide?
Actually, all three survivors will be tried on the charge of "murder by request" on the assumption that all were accomplices to the series of criminal acts on the day of Mishima's death, which include assault, wounding several SDF officials, forcing officials to act, and unlawful incarceration.
It seems the sentence was indeed light. Koga was sentenced to four years penal servitude, but was released a few months early for good behavior.
As far as I can tell, there have not been any challenges to the law since.