I could see this phrased as "You are defenseless, hand over your planets peacefully so we do not need to butcher your people" or something along those lines.
Yes, it would require some specific circumstances (enemy fleets almost all destroyed, legions reduced X amount or %), but an argument could be made that some leaders would rather spare their people in such a situation than subject them to the horrors of a war that seems already lost.
Insofar as asking the AI to quit the game, would you stay in a game if the situation described above was yours? If so, then you are much more committed than me. I'll start a new game before it gets that far.
