Thinking about it again, I'm not sure there is so much need for it. And it might actually be broken.
I could only see a couple of spells that would make sense to tattoo. Armor is the obvious choice, of course. After that, it starts to get a bit problematic already.
"Always on" maneuver spells? Aspects are already always on. Would it be a spell you can just use over and over again? Then it feels more like it should be a stunt, since it grants a constant bonus equal to a fate point.
Attacks? Based on the 8 shift spell you propose for the Armor, you would be able to get a constant weapon:4 for the price of 1/4 of a refresh. That's 8 times more powerful than Claws, and besides a bit of ink under your skin, it doesn't really come with much of a physical and optical downside.
Skill replacements? Sure, those would work, but thats a whole lot of skills you get to have at a constant value of +4.
Let's take the 8 enchanted items and your base value of 8 shifts per item. The tattoos cut it in half, ok, so it's one item with Armor:2, another with Weapon:4 and 3 skill uses at +4. Since you will most certainly take skill replacement tattoos for skills you don't have, and even 3 applications is a lot, if you have them available at all times. That's 3 free trapping moving stunts.
I really think it is better to take those things as powers, not as enchanted items. If you don't want to take toughness, just grant your tattoo guy an armor bonus like you would anyone with an armored vest, just without the vest. I think the impact on the game should be minimal, most of the time. Claws and Breath Weapon will do for attack spells, or the equivalent powers from the custom powers list. Skill replacement can be done with a tweak on mimic abilities for direct skill replacement or one of the minor powers, depending on what you want to do exactly.