You're walking a thin line here. In most cases, chicks are going to side with their chick friends when a guy is involved. Especially if they are roomates.
"Do you realize in the entire history of western civilization no one has successfully accomplished the Roommate Switch? In the Middle Ages you could get locked up for even suggesting it!" Hey Drex, my advice is to talk with the roommate about it. If you ever find yourself alone with her, just tell her you think your ex is just putting on a charade and what not. Then say something like "I'm going to have to talk with her about this because I can't really take this crap anymore, yada yada, if she really wanted to be my friend, I wish she'd be act more like you."
I heard a female speaker speaking to a group of women point out how petty we can be. Often it isn't that we want someone else's husband, boyfriend, children, house, job or general happiness, it's that we don't want them to have it either. I think she'll hate her roommate and she'll hate you even if she isn't remotely interested in you. This isn't always the case, there are quite a number of women who might be just fine with it or at least understanding, and a number of women who might simply be a little jealous. But you said she can be rather cold towards you when she's around her friends. If she isn't generally in the practice of cultivating love, forgiveness and compassion towards you, she'll probably just hate you.
Why don't you just get together with the roommate without the ex? No need for a confrontation or even a conversation. Just a shift in attention.
