This happened in our last game.
Our keep liege has these rights:
You have the right to impose law on the villages under the stronghold’s protection.
You are a legal heir to a crown. Upon its fall, you have the right to contend with your fellow heirs to claim its
inheritance.
You have the right to give justice to the people within your holding and your protection.
At the end of the heated battle the King of Emory (the king of the lands our Liege is heir to) is set upon by two of our PC's. The Keep Liege and the Troll-Killer both approached the king and wanted single combat with him. I gave initiative to the Troll-Killer. The Keep Liege declared her rights denied. She had the ability to impose law here and this was the crown that was hers by right. The Troll-Killer acquiesced after her heartfelt speech and let her join in single combat with the (soon to be ex) King.
Technically. She had no right to challenge someone to single combat. She had no right to kill a king. This was either an implied right that her other rights gave her, or it was a right she just didn't have.
I don't think as MC the principles would allow me to say, "You don't have that right." I believe this would be wrong on a fundamental level. If a player believes that logically they have this right as justly follows from some other right then it would be poor form of ANY MC to stand up and say, "NUH UH!" Any arguments the players have can happen in character and be AWESOME! I'm a fan of THAT.
It certainly needs to be open information known to ALL players what rights the player ACTUALLY has. Arguments can flow from that. I don't think it's up to the MC to tell a player if they can or can't trigger that move. It makes it a very unique move in that the MC doesn't have ANYTHING to do with it. The players trigger, enact and resolve the action without any MC interference.
Let the eruptions of gamer angst begin!