For me, it would be enough to start as a team with one player (already possible), have shared vision with him (minor tweak?) and have my friend play as my teammate (via ctrl+shift+z cheat), no need for different diplomacy, when i will do trading with my friend, computer diplomacy as it is now will be fine (we will just use gifts)... and order of things could be:
1. I play "default player" empire, (then ctrl+shift+z and we go to part 2.)
2. My friend plays "his" empire
3. All of the AI players play, and me and my friend look what is happening (allied vision. No need for replaying AI moves)
In DA we will be able to customize all AI players (and the one that my friend will be using), so all we need now is two things:
a) shared vision (I don't know how hard is this)

on step 3, the AI must play all empires except mine and my friends! Maybe this can be solved like this: AI just start playing with first empire available (the one after my friend, i think it is already like that), but one minor tweak: before AI starts to play, it reset "player control" to default player (ME), so i don't lose my turn.
I'm hoping that this is not more than a one-hour-job, and someone will make my prays come true (and list this as undocumented feature until real multiplayer arrives). If it is a big-time-job, then don't bother, I will wait for multiplayer expansion!

No hard feelings!