Hmm. Interesting idea. I wonder...
I like paradigmx's idea of "expeditionary forces." That sounds pretty neat, actually.
I also understand Ahriman's concern that the AI wouldn't be able to handle the mechanic. We could eliminate one exploit, though, by having units you bought from a nation revert to their original owner if you declare war on that nation (this could represent them getting kind of pissed that you declared war on their homeland).
Then again, the trigger would have to be the nation buying the units declaring war, otherwise you could just sell units to the AI, invade them, and then get your units back. This is a bit confusing.
Maybe my brain will work better when I'm not so tired...