Hang on a minute... Won't law enforcement be looking for these tattoos eventually, and even while on the surface they can't be directly linked to illicit activities aren't all block chain transactions nigh impossible to be forgeries? Admittedly, I'm on the outskirts of the whole crypto market but this seems like it'd be able to traced with a warrant to search the electronics of both parties involved if both of their 'wallets' were uncovered.
Whether or not someone would even bother going this far on a prostitution charge is up for debate but some people are fucking crazy man... I might be totally wrong in my assessment but this seems like a really bad idea to me.