No, that's impossible. There's a script that can change faction, so you can e-mail him the savegame, he runs the script, plays with his faction sends it back you run the script play a turn as your faction,... However your battles ofcourse can't be played against the other human player etc...

Note that this isn't a project of EB or anything. I don't even knwo if it works with 1.5 and or EB. I don't even know if it's finalized. But you can do a search and try.