According to Ask A Mortician it is legal to tattoo a dead body, but it will NEVER heal. I’d assume that the legal part is subject to state laws & the willingness of the tattoo artists would also come into play.
My thought on the NEVER healing is WHY does that matter…once buried/cremated, NOBODY will ever see it to know the difference!!! I think that I’d suggest that they get the tat BEFORE dying since it means so much them…damn the torpedoes full speed ahead type of thing!!! UNLESS they’re still healthy, they might want to wait on the tat until they are actually given a certain length of time left. It seems like a very strange request to me, but I can see all kinds of problems arising out of the request!! You might want to have a legal document drawn up to cover your butt IF you’re really thinking about doing it after the fact.
Another thought, you might seriously consider one of those fake tats the little kids love so much. The mortician could apply that one & it shouldn’t be considered defacing the body. Then again, you might have problems finding what they want. Maybe that they insist that they talk to the tattoo artists because it’s so important to them.
Another question…how are they going to know that you didn’t have it done once they’re gone??? I’d let them leave this plane thinking their wish will be done & then go oops after the fact.