Sorry for the minor thread necromancy, but I just read this after following the link from the farming thread.
I really like some of the ideas here, especially your particular brand of agents as the primary means of outside interaction. I feel that it fits in really well with the current character, skill, and task based system that makes up a large part of DF. The vast bulk of the mechanics for this seem like they're already in the game, and it's just a matter of providing things for agents to do, plus an interface to send them out.
Things to do will come naturally from Toady's already stated near future goals for Caravans and Armies. For example, once food shipments and starvation continue past worldgen, it may be possible to send an agent out to hire bandits to starve out rivals, or perhaps poison their shipments. This could weaken future invasions, or perhaps leave them in a weaker bargaining position on the diplomatic table (another obvious place agents would be useful for). And with armies traveling the world map, you would want to have spies tell you where they are, what their composition is, or perhaps raise guerrillas to bloody them up a bit before they reach you.
In short, I really like the idea of agents as the main mediator as you expand past fort level management, and I feel that it's a natural extension of the current game state and it's near term plans.