Yea, I got a freebie castle I didn't even ask for, because I'm married to Ira and fighting for the Southern Empire.
If you marry the child of one of the pretenders, you honestly don't even have to think about the main quest. The Empire will go to your children by rightful inheritance so who cares about some dead guy's flag? Besides until management of fiefs and kingdoms is actually interesting playing the vassal game is just fine.
Bottom line, if yuou want a fief, either butter up the King, or butter up a powerful noble clan. They'll eventually come throguh for you if you keep voting with them.