Robert: You can make diplomacy sending a nameless envoy. Like Osoerde you might be commanding troops in the crusade or have any other reason that your regent can't come. So sending a lieutenant, other able assistant or a vassal is perfectly fine by me.
Since I don't want to penalize people for being active in other corners of the world but mine, I choose to say first come first served, weather you come by regent, lieutenant, able assistant or vassal.
And then again there are people, and important people... I would have saved a seat for both Boer and Avan if they hadn't answered quickly enough.
And yes, the "seats" are the ones I spend a court action to include in a court session. Others are perfectly welcome to attend and state their point of view too.