Perhaps have it so that a Knight must be under an oath of fealty in order to have a vote? That way there's a little more incentive for the bottom rank to swear fealty as well, and higher ranks can trade giving a Knight a vote for him voting along certain lines.

Say a knight favors CA 1.4 and needs to get a vote while a Duke expects to need one more vote for CA 1.7, he'll take that knight into his service for a minimum of the duration of the GB session so that the knight then gets a vote, and the Duke scores his extra vote. It would increase the political flexibility of the lowest tier of the system while also punishing them for not having a province, but making it possible for them to participate if they so desire.