I would guess it might not work in their system. Main gear pieces don't just equip onto your character, they replace that section of the body model, including bare skin. So the tattoo would be part of that model.

Im pretty sure jewellery just sits on top of the model, so it might not be able to apply tattoos in the same way - they might float over the skin or do odd things, or just not fit properly. (I'm only guessing, but I think there might just be one shape used for all races.)