A question about slots: have you written this according to 5e rules? In 3.P and earlier, a tattoo on the hand would take the hand slot (in place of a glove or the like) or maybe the ring slot, but thete is no weapon slot as such. Likewise, a tattoo could take up the torso slot, but not armor.
In general, cool idea. I've had this idea for some time but never thought it out this well, starting with a particular item that I want for one of my characters. (Ishould writehave written that item up in the 1001 Homebrew Magic Items thread.)