I was fairly sure from memory that the one in the black & white strips with the mirror wasn't her.
I've just rewatched the documentery, and I'm pretty sure that in the rehersal footage she is one of the girls (there are two) wearing black leggings and a white top - she's the one whose top/t-shirt does not have a collar. The one who in that first bit of the rehersal scene is holding a large feathery fan. Even in the rehersal scene her red hair is pretty touseled. Just at the end of that first clip (before it cuts away) she gives some little look/movement, something about the way she turns (which I seem to think I recognise - having seen her on stage several times).
A little later there's a moment on the documentary where Jennifer Connelly doing a voice-over (picture the ballroom rehersal) saying "I'm awful at ballroom dancing", just before it cuts to Jennifer herself, I'm pretty sure that Louise and whichever guy she was partnering in that bit of the scene move down to the bottom left-hand corner of the screen and then just out of shot as it cuts away.
I have also just watched the ballroom scene again, I'm not too sure about this, but I think Louise might be one of the dancers quite visable throwing herself around at the very end of the scene, when Jennifer smashes the glass.
I'm pretty sure I've spotted her in the rehersal scene. But I'm much less sure about the Ballroom scene itself.
Hope that helps
Emma