Personally, I like the whole tattoo theory--after all, Ryuuji's father has more than proven himself to be a downright twisted son of a bitch, and he may have done it to Ryuuji with the (possibly hollow) threat of marring his face further when/if he ever loses, so as to make Ryuuji further resemble his father and remind him of what he's fighting for. Of course, the "just regular eyeliner" and "covering up a scar" thing works for me, too; I'm not picky.

I also like the "blood-magic scars" thing, because I'm weird. Again, Ryuuji's father has him twisted into something (that is, at the least, mentally unstable) terrible, so Ryuuji might think nothing of carving runes or whatever into his flesh to work a spell. But the wrist-blade theory (which I never thought about :x) works--he IS a celebrity, and being that he doesn't keep any body guards around him, he must have some other way of defending himself.

... Yeah. He was home-schooled all the way. XD

I mostly go by the physical/mental/emotional abuse, but I can see where the sexual abuse comes from. Ryuuji is absolutely gorgeous, and his father is hideous. Not only that, but, for example, in some of your fics about Ryuuji, there are mentions of Ryuuji being willing to use his sexuality in very unhealthy ways (drugs, manipulation, etc). Maybe he got that way by "virtue" of his father? I'd personally like to see a well thought out fic (even though it would be extremely squicky, even if there were only mere mentions of it) about it.

And for my part, I would love to see a part two. XD