Based on the Inscribe Magical Tattoo item creation feat's description, it seems like one should be able to make almost any normal magic item into a tattoo equivalent simply by doubling the cost.
Given that one is effectively paying for the item to be slotless, are there any rules to suggest that such a tattoo item must occupy a slot that corresponds to the one occupied by the normal item it is based on? (I could see a logic/fluff argument for this in some cases, less so in others.)
For example, could I put a "hat of disguise" equivalent tattoo in my body tattoo slot?
You might ask why this even matters, but it might matter since some tattoo locations are more conspicuous than others.