Haven't checked this thread in awhile. Having played my Last game where I've been the MEE for better than half the game, it's become obvious that once I reach that status, there is no no way the AI can make peace with me, even if their disposition is brotherly. I tested this out by keeping a couple of AI opponents brotherly for many, many turns, to see if they would offer a treaty, and they didn't. So of course I annihilated them.
Anyway, the explanations in this thread seem consistent with what I've seen. Thanks for the info.