From my reading of the rules here:
http://www.d20pfsrd.com/feats/item-creation-feats/inscribe-magical-tattoo-i tem-creation
it seems like it one should be able to make almost any "normal" magic item into a tattoo equivalent simply by doubling the cost.
My question is this - 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, if I made a magical tattoo equivalent to say, a hat of disguise, could I put it in, say, one of the "ring" tattoo slots, instead of the "head" tattoo slot?
There are some things in some books that require a thing be visible to gain its benefit but other than that, the answer is that it doesn't matter where you put it. One example would be Shoanti War Paint, which says it can be applied to visible parts of your body. Another would be Hypnotic Tattoo which only works while visible.
If the magical tattoo said it had to be placed in a specific place to function then it would. If it doesn't, it doesn't. The page linked above specifies that the tattoos have to be placed on a part of the body where a magical item could be worn, but it does not say anything about it corresponding to the item type. Therefore, you could have a magical tattoo duplicating a ring's power inscribed on your foot, or a tattoo duplicating boots on your wrist.
It specifically says you can't inscribe the tattoo in the armor, eye, headband or shield slots, but otherwise anything goes.
TL;DR, yes, you can put your disguising tattoo on your finger. =]