I'm inclined to agree with CMEast on this one. Not only can the tattoo actually be marred or obliterated, but it was mentioned that there wasn't anything to enchant. To that, I say: What about the ink? Or the needles? A magical tattoo could work as either an enchanted or focus item, I think. The idea of an old Yakuza member covered in tattoos that can summon power, I think, makes for a good character. Certainly, it could be kept as scene dressing for his magic, but I don't seen anything wrong with the idea of magical tattoos, myself.
If there was really the concern about them being harder to take away (although, I would argue, that Harry's rings are, really, no "harder" to remove...at least while he's conscious and resisting), and since the whole concept is sorta house ruled from the get-go, why not just make it cost another enchanted item slot? Therefore, an enchanted item tattoo is the cost of a focus item, and a focus tattoo costs 1.5 focus item slots. Yeah, it could get a little unwieldy with a small bit of math, but it helps offset some of that potential "abuse".