In the meantime, I'd prolly be content with indies set to be always on patrol mode... and if/when they'd lose to a superior force, the survivors would retreat to the castle and defend it normally.
Perpetual patrolling would have the added bonus of making it harder to defeat indies by assassinating the commanders.
The downside, as someone pointed out, would be some lost population if something generates unrest in the province. IMHO it wouldn't be a big issue though, merely a drop in the sea of all these pop-destroying events.
