Also, I wasn't clever enough to think of it, but R.A.B. = Regulus A* Black, yes? Are there any other possible contenders?

It's the logical choice to me, if only because JKR can't --I'd say, in good conscience-- introduce anyone else at this point. You'll notice the only new playing characters were Scrimgeour and Slughorn in this book, and the book could've gotten along without them, really (not knocking them, just saying). If RAB=Regulus, she's already got a partial background, a motive or three, and the ability to bring him back to life, without wasting too much paper to get him integrated into the story.