I know somebody who works at law firm handling mnta's side. This guy isn't working on the case but knows of it. According to him it's supposed to be part of some public record so there's no insider stuff here (he's a lawyer!), but I don't have a link. My next step will be to ask mnta's ir group. Let me know if you hear anything and i'll do the same.
Check the ihub MNTA message board (it won't let me post)... Someone posted about it so looks like you're correct that the SG responded, but doesn't appear to be public. The ihub poster makes the point that perhaps the SG believes MNTA is the better case for supreme court decision, but without the brief, there's no way to tell until it is available.