I want to mess with her, but mainly just because she has revealed a weakness (relationship insecurity) that seems too delicious to pass up.
My plan is to "confess" to her in a simulated drunken overshare. I'll tell her that I have often wondered if I could "also" seduce her fiancé. Depending on how much she has had to drink and her current level of paranoia, I may have to wait just a bit to let that thought have its full effect on her (which given his varied and prodigious sexual history should be a pretty easy sell). After she has let that marinate for a while, I will then try to seduce her myself while she is (hopefully) vulnerable from the thought that her fiancé is cheating on her with all of his smarter-than-she-is-work-friends.
I think chances of it succeeding are pretty low, but chances of it increasing her insecurity are pretty good if she's so thrown off by a Halloween party "sexy third world slave" outfit, that's she's basically prohibited him from ever seeing this woman again. (I wasn't there and there apparently isn't any photographic proof of whether or not a third world slave costume could be considered "sexy" without seeming really grossly imperialistic and in poor taste -- this is just what I've been told).