Stormrunner
Explorer
I'd allow it, but certainly not for free!
Mechanics-wise, this is equivalent to allowing a wizard or sorceror to cast spells without needing material components. If Eschew Materials is important enough to be a feat, then Eschew Focus should cost a feat as well.
If you feel that's too strong, then you might rule that the tattoed focus takes up an item slot - a tattoo on the face might fill the "mask/lenses" slot, one on the hand might fill the "gloves" slot or a "ring" slot, and so on. However, this limitation is a little easier to powergame around by making items of different shapes (e.g., bracers of protection instead of ring of protection).
Mechanics-wise, this is equivalent to allowing a wizard or sorceror to cast spells without needing material components. If Eschew Materials is important enough to be a feat, then Eschew Focus should cost a feat as well.
If you feel that's too strong, then you might rule that the tattoed focus takes up an item slot - a tattoo on the face might fill the "mask/lenses" slot, one on the hand might fill the "gloves" slot or a "ring" slot, and so on. However, this limitation is a little easier to powergame around by making items of different shapes (e.g., bracers of protection instead of ring of protection).