Civil Disobedience? Banishment/exile as a result
Protests aren't nearly as effective as they should be because they require fellow nobles to support you. Silent protest doesn't change anything and public protests usually result in the same as civil disobedience.
In the case of a duchy change - That happened in the war, the duke was marked a coward (and also disappeared from the game nearly the same day as the change) and was shat upon within the realm for doing so without making an attempt to any of the above instead
You bring up rebellion, but such an action requires support - usually from the outside. Outside support requires something in terms of payment. Future promises are meaningless when you're currently locked into a war with the same people they want to rebel against. Information pays off now
While it's an irritating course of action, spies serve their purpose. They're faster and usually more effective than the above. Had we been able to completely block the Barcan army without significant losses, this war would have likely ended where it started. In the time i've been playing, i've only seen the occurrence of straight up treason like the Duke once, and shared military information twice. At this point the only information I receive is letters from nobles voicing their discontent and the wish to leave the war zone unmolested.
