Milhouse, actually he wrote that if one or two women come to a shiva house the men can daven there without a mechiutza. He did say what you said about a derasha but only lechatchila (OC 8:11). I still don;t undertstand what difference an invitation made. Maybe he thought that since the derasha was meant for kiruv purposes the women might not dress properly. One could say that כלל הציבור means that the general public would come.
BTW, I asked a rav who is a talmid of Rav Soloveichik and he said that RS’s motzaei Shabbat shiurim in Boston had mixed seating. He did say that when Rav Schachter came to speak at his shul he said that he “preferred” separate seating but did not say that there should be a mechitza.