I don't think that will work. Too easy to exploit in my opinion.

For example... Say random_barbarian_sneakybastard amasses a 20-stack army at a weak point along my border. Let's also say that the AI decides to actually use that army while I'm caught with my pants around my ankles instead of just sitting there like a twit, as per usual. In this scenario I should lose a province or two while gathering an army to deal with the unwashed pigdogs. And rightly so for leaving my border undefended! However, with the modification detailed in the post you linked, all I'd have to do is assassinate the family putz leading the swine...and 3000 barbarians get stuck in the snow overlooking my nice, warm, undefended city...chock full of food, plunder and hairy greek women...unable to command their own feet without the aid of a general.

I like the concept, but I really don't think it will work. Now if you can figure out a way to give a leader-less army the option to retreat to the nearest friendly city instead of just standing about... Or, if they're within sight of a poorly defended city they could go a-maraudin'.