If you give tattoos a similar restriction as focus items have, you can have them as focus items, sure. Maybe make it so you first need to touch them with your fingers, drawing the power out of them to cast the spell, things like that. People in the know can put duct tape over your tattoos then, paint them over, bandage you, tie you up so you can't touch them, etc. Those not in the know, who would not take a wizards amulet or bracelet, would not care for your tattoos as well. They might still tie you up, but a good athletics roll could allow you to stretch far enough to touch them for one spell to free yourself.