Very interesting observation! I hadn't seen the tattoo in the promos. The minigolf pictures were taken on on August 9, 2005, the day when the third episode of S7 (Reese vs. Stevie) was shot, so unless Frankie is wearing long sleeves throughout the rest of the season, or used heavy make-up, there should be signs of it.
So what do you think? Was it there in Burning Man, hidden under make-up, or not?
In this interview Frankie Muniz explains that makeup is often used to cover tattoos: "The makeup is so advanced these days, you'd be surprised how many actors have tattoos! It takes some time to cover them, that is the bad part."