Sam2,
Interesting. When I asked a shaila about this it was a (potentially) very b’dieved situation. I still got the impression, however, that m’ikar hadin it was fine. But that could just be an impression.
My main point however, is that even those that don’t allow it, i.e. the Behag, aren’t arguing on the Gemara. Rather they are understanding the Gemara differently than Rashi (and other Rishonim). The Gemara doesn’t say explicitly that women can be motzie men, it only says that women are obligated in megillah.