I read your blog post, and I agree with much of what you propose. That is similar to the way I chose to handle it prior to the publication of AV2, which has its own rules for magical tattoos in it. The AV2 tats are counted as another item slot. There are some interesting ones, especially if you get linked tats for the entire group.
For my own group, I allowed a couple ways to get magic tattoos, even before AV2 was out. They can be the equivalent of any item, and occupy that slot. That makes it harder to lose the item, but also harder to upgrade it. So the fighter in the party has a cloak of the walking wounded tattoo, in the form of a shield with three blood drops on it, at the base of his throat. Another use, is as a means of gaining a familiar. Your familiar is a tattoo that comes to life when it is active (and leaves your skin), and returns to its tattoo form when it is in passive mode. You take the familar feat as usual.
As for piercings, I would not make them into a different item slot, but would instead offer them as a viable choice for the existing slots. Any facial piercing uses the head slot, and can have the kinds of enchants that occur on head slot items. I'm not sure I want to think too closely about piercings anywhere other than the face.
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