I don't really see the point. If your already playing as an ally to another player, then surely your capable of deciding on a pre-set victory condition amongst the rest of your group?
Unless you have a real hankering for the ascension message
The problem with the suggestion is that your going to have players who want to ally but not have their resources totalled up, or players who want alliances in the game, but don't want allied victory to be possible, and a hundred and one other potential victory situations being requested.