Idea from a thread on enchanting monk's fists. Someone mentioned magical tattoos that take up item slots. (The magic of the tattoos interferes with magic item use.)
I don't have the rules on Shaonti face-paints from CotCT in front of me at the moment, but it's something similar, IIRC.
Not sure what Seoni's facial tattoo does, but I keep coming back to Second Darkness' Cyphergate.
Anyway, I don't see where this concept is broken. In fact, unless you can take the tattoo off (or have interwoven Celtic tattoos, and choose which one is active [like a staff... ooh, I like my brain]), it is actually LESS unbalancing than magic items themselves. I'm pretty sure that Permanency is cheaper than the equivalent magical item/tattoo.
But back to the main idea for this thread, for each character class, what tattoos would you want?
For Shaonti barbarians:
Eyes - something allowing eagle (distance) vision
Face - wolf pattern around nose for Scent ability
Arms - +STR
Armor - natural armor (like barkskin, but re-themed)
Amulet - endure elements
Feet/legs - something similar to horseshoes of Zephyr, increase movement rate
For arcane types:
Eyes: detect magic, arcane sight, read aura (forget the spell name)
Head: comprehend languages, +INT, or Pearl of Power
Hands: +DEX (for ranged touch attacks)
Feet: expeditious retreat
Belt: feather fall
Cloak: fly
For divine types
Eyes: detect [alignment], deathwatch
Head: +WIS, otherwise same as arcane
Hands: healing spell (as Pearl of Power), possibly Light as well
Amulet: Status (also Pearl of Power)
Honestly, there's a LOT of potential here.