I think the better option would be to give robe privileges to whoever has the most kills across all factions. Like, one person at a time on the server.
While I'm sure there will be a better thread to make this criticism in later, I have to agree that OCB needs to be incentivizing smaller groups, not larger ones. The successful 3-man faction is more deserving of the reward. While I get Az's vision of big factions made up of many small groups, that is just plain unrealistic--advances in the tools we use to communicate outside of the game have made this a thing of the past. Skype makes it incredibly easy to form groups at the drop of the hat. To combat this, players need to be either forced or heavily incentivized to be in smaller groups. Giving special rewards only to the big alliances is a mistake and a step in the wrong direction.