I think Nauro has had the best idea. An easy AI, that everyone should be able to beat, but will still put up enough of a fight so that you wont be losing a battle that you were winning when you DC'd.
Another idea spawning off of Nauro's. (That I had.)
Unit's of the DC'er do nothing but get a slight/major defensive buff so they die more slowly. Giving the person the chance to get back in game. But not stopping the person who is still in the battle from winning.
The biggest problem with this sort of thing is: "Oh, I'm losing?" /unplug modem. Now I'm griefing the person who was beating me by forcing them to take longer.