[thejeff]

As I said, you could house rule it anyway you wanted. That would be a minimum and it's certainly abusable. Requiring announcements is certainly reasonable, though it raises the question of definition: What has to be announced? Since the formal "alliance to claim victory" only matters once you're within shooting distance of victory conditions, do you have to proclaim other forms of cooperation?

Working together all game and only admitting it to claim victory is a possibility, but so is fighting all game and actually making the alliance at the last moment to win the game.

Would also allow interesting possibilities for betrayal. With the right timing you could throw the game by switching alliances and never even have to fight the people you'd betrayed.