My biggest fear with reactionary events especially late game, is it forces you to a military type victory goal every time. Maybe options at game start to include or exclude type of events could over come this?
Interesting idea! No, it shouldn't be difficult in fact that's what "surrender" in MoO 2 does. It was rare but a heavily beaten AI would surrender to somebody basically making an union like you described (except technology wouldn't be transferred, that made me sad when I wasn't playing creative).
I would hope reactionary events would also be scientific, such as you must research a new tech to defeat the borg or you will get your tush womped. Or they could be build a new wonder such as a stargate or a blackhole generator.
Yes, I remember that event firing once or twice in my early games of Moo2, sadly I never benefited from the surrender though as both times the factions surrendered to a third party. (interesting that on the harder levels I never saw the event fire, maybe it was just me) Now if an event like this, which already has a history, could be polished somewhat it could present a serious challenge. At the very least you should be able to cause the player to pause expansion while he/she assesses the situation anew. This type of an event may even address Thrangar's concern and actually make a military victory harder to achieve thereby causing the player to explore other options as well.