Though, I can't imagine the other idols wearing Black Rock Shooter's outfit.
I totally can, that would be super fantastic. They could even make it color specific to each idol if they so wanted, like so:

I do find it a bit strange BRS hasn't been in Project DIVA, though it may a it-has-it's-own-game-on-PSP deal. Where BRS was developed by Imageepoch, a company other than SEGA. NBGI is workin' on a Nendoroid game with BRS in it, so who knows what may come? Sony and Vocaloid have a pretty close relationship, I suppose anything's possible. I'd be up for it, song and all.