I think this idea has potential, but it's too early for it. When the world becomes more dynamic and interactive, and the player is able to move fortresses around (either by actually relocating in fortress mode, or by starting a fortress with the companions in adventurer mode), it should be revisited. I wouldn't like to see a half-assed version implemented before the world can react suitably.
Nor would I want to see it turn up before the player has more options than just being the loser in a surrender situation. Why shouldn't the player be the invaders forcing a surrender (or, for an especially terrible fortress, the defender forcing the invaders to submit)? Why shouldn't the player receive news of distant battles and lost or gained territory from visiting diplomats, with an option (or requirement) to send along help for a war effort in the form of personnel, materiel, or both?
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Dwarfs surrendering is pretty easy to imagine for me. Civilians already panic at the sight of small birds and other animals, sometimes those that don't even have a valid path to reach them. All it really needs is a civilian smart enough to understand that the interrupting critter is between him and safety, and that running off into the wilderness isn't going to help.
Urist McFarmer cancels Gather Plants: surrendered to badger.