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.
Best Answer
The feat Inscribe Magic Tattoo allows creation of magic tattoos; the feat isn't a wholesale replacement for other item creation feats
When designing custom magic items, first compare what you want to create with extant magic items. In addition to those tattoos listed in the callout box alongside the feat, tattoos that require the feat Inscribe Magic Tattoo can be found here. So, while ill-defined and given only a short list, new magic tattoos should follow published guidelines and not just be considered another means to acquire a semislotless wondrous item.
That said, the GM can in his campaign can allow anything, but, conservatively, it's probably best to keep tattoo magic items associated with their item slots according to Altering Existing Magic Items. However, since you're already paying enough to create a normal slotless wondrous item--and could've just created such an item with a typically more versatile feat like Craft Wondrous Item--, having double-price tattoo boots on your torso shouldn't make any difference to the game.
(By the way, the demon senses implant establishes a precedent to use the feat Craft Wondrous Item to create actual slotless items that are probably as hard to steal, lose, and destroy as magic tattoos. Just sayin'.)