I think tatoos could be accpeted as Focus items. Sure it is a lot more cheesier than a normal focus item but you need to remember that when magic goes bonkers, your foci will be absorbing some of the damage. In case of tatoos you will loose both the foci and eat the damage.
Secondly, there is a limit to amount of magic an item can handle. a magical focus tattoo should take up a lot of space, As a rough count I would say Hand and forearm (x2) Chest, the whole back (in yakuza style), Face and Head (shaved) count as slots, each slot (except back maybe) gives you enough room for +2 worth of effects
Then there is the Potion slots... You "did" allocate potion slots for the ink you created as base for your tatoos didn't you? And you did use good ingredients like tears of a willing virgin, your blood, the bones of mountains?
Lastly there is the magical sign. The more capable a magical artifact is, the more obvious it becomes. So a single point (Control, Complexity, whatever) worth of tatoo could be exotic but not too obvious but a 4 point one will be obvious to even mortals, let alone supes.
I hope you are fine with going commando because I can see a lot of damaged/destroyed clothing if you are an offansive minded wizard who uses tattooes
