Yes, that would work well, maybe. Depends. The king sometimes still selects the duke and can still force secede him. There might also be things to iron out so a stupid duke (and they're quite common mind you) doesn't get all trigger happy and keep fighting other duchies for the hell of it while a foreign army smack them to the ground. Or at least don't make it too easy to use it as only for subversive means.
Although, if you mean that duchies can fight foreign duchies (which you are), that would be great. For duchies in the same realm, it should be considerably harder, and there should probably come with penalties, so that if you must go against each other, you better have good reason for it.
