Is it legal to tattoo waitpersons’ hands?

I recently put a blog post up about how to get important messages across to staff. In it I joked that a restaurateur might put “tattoos on waiters’ hands”, but that that probably wasn’t legal.
Today I got an email from a waitress , (I will call her Avril), who said this is exactly what her boss did. He made the staff put temporary tattoos on to remind them to do certain things.
If the waitperson refused he or she would be allocated worse shifts.
Avril sent the enclosed picture of her boyfriend’s hand, (he has the job of answering the phone as it’s at the bar and he is the barman.)

 
The scary thing was, Avril said that her boss had offered $100 to anyone who got a permanent tattoo to save him the expense of the temporary ones!
Any one else heard of this practice?