Tattoos would be difficult from a design perspective because of the limitations of HS2. From a fluff perspective, they're also
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. For older women this isn't a problem and you may have noticed that Raquel has a small one on her abdomen. Piercings are easy, and you may have noticed many girls in the game wear earrings, though again, schoolgirls aren't allowed body modifications.
Hairstyles are something I've messed about with, players who've been around longer may have noticed that I changed Stacey's hairstyle from the one she started with as I didn't like it.
Slaves are already identifiable by the collar they wear which can't be removed, I haven't established that slaves are tattooed, but because Mistresses can do whatever they want with them, there'd be no reason an owner couldn't decide she wants to decorate her slave in a certain way. Sucks for the slave if she doesn't like it!