I think we have an answer....
MA: R.A.B.
JKR: Ohhh, good.
[All laugh.]
JKR: No, I'm glad! Yes?
MA: Can we figure out who he is, from what we know so far?
[Note: JKR has adopted slightly evil look here]
JKR: Do you have a theory?
MA: We've come up with Regulus Black.
JKR: Have you now?
MA: Uh-oh.
[Laughter.]
JKR: Well, I think that would be, um, a fine guess.
MA: And perhaps, being Sirius’s brother, he had another mirror -
JKR: [drums fingers on soda can]
MA: Does he have the other mirror, or Sirius’s mirror -
JKR: I have no comment at all on that mirror. That mirror is not on the table. [Laughter from all; Jo's is maniacal.]
MA: Let the record note that she has drummed her fingers on her Coke can in a very Mr. Burns-like way.
[Laughter.]
~~~~~~
So Regulus is a "very fine answer"
Now how does the mirror fit in?