Hey, if I'm right on this one, feel free to quote me when it's finally revealed, but Simone has kinda played this game before, with the mystery Batgirl in Birds of Prey, before we learned it was Charlie.
Given Didio's said no to using the Multiverse for a while, and hasn't shown much interest in the New Gods, beyond anything Morrison has planned, I really hope it is Knockout, but I doubt it. That said, Simone is one of DC's better writers, and could probably twist Didio's arm if she wanted to.
I would love it to be Knockout, frankly. But then, would she be a member of the Six? Their numbers are full, after all..