Firstly: I agree that the yea or nay on tattoos/bodyart acting as focus or enchanted items is really a 'house' rule. Just a couple things I wanted to point out to others.
Secondly: If tattoos are allowed as 'items', then some determination as to size their effective size would need to be made. In YS page# 281, there is a chart in the upper right corner of the page which shows how many focus or enchanted item slots an item can have, depending on the size of the item. Given the examples used a ring, fist or rod, basketball or staff, it would suggest that just having a large surface area is not quite enough.
Thirdly: It was suggested that the needles used to ink the tattoo could be considered part of the item that is being enchanted or attuned. Here I disagree, the tattoo needles could be focus items for Thaumaturgy (Crafting) specializations. However, the tattoo needles are used to make the tattoo, they are not part of the tattoo.
Lastly: In one of the later books (Small Favor? I don't remember exactly which one at present) Harry was cleaning his duster, and there was some mention of the magical symbols he'd inked and etched into it using tattoo needles. In short, the symbols and designs, along with the intent were used to make the duster itself enchanted. The symbols by themselves were not enchanted, they were part of the enchantment. A tattoo to my way of thinking would be the same as one of the symbols Harry used. So the choice would be to allow a person to enchant their skin, or have them enchant items separate from themselves. Since tattoos or a person's skin, is not really a separate 'item' from the person, that is why I would not allow a person to use tattoos as focus or enchanted items. In a mechanical sense, allowing a person to tattoo a focus onto themselves would effectively bypass the limitation on specialization bonuses in the description on Refinements on YS page# 182, with Refinements, additional specializations can be added onto elements, but it follows a tree similar to the skill tree, i.e. before a caster can take a +2 Power bonus to Water, they also need to have a +1 bonus to Control (Water), or any other specialization, before Power (Water ) can be expanded. By allowing a caster to essentially have their body be their focus, they can ignore that restriction.