First of all, can referendums be limited to a message group? I recall seeing a feature request about this but did not find it during my brief search.
Regarding the suggested system, I would do it differently. I am no magistrate though and can not say if the proposed system or my suggestion are IR compatible with any authority. I feel that both are.
The Rule:
All region lords, dukes and council members are eligible for a seat on the senate. Should you hold such a position and wish to join the senate just let us know and you will be made a member. As a member of the senate you are expected to contribute your voice to the discussions and decisions being made. Should you wish to leave the senate at any time simply state your desire and you will be removed.
Optional: At a minimum, you, as a senator, are required to make a senate wide statement regarding each and every referendum brought before the senate.
The Culture:
With a basic rule in place, begin molding the culture. Be vocal yourself. Specifically call out those senators who have not contributed to a conversation. Encourage all others to do the same. If the referendum “Should we eat cheese” is brought before the senate and only two out of five senators have announced their positions, directly call out the other three and ask their opinion. If they still do not respond, send a private message. People will sometimes give an opinion if asked directly even if they were not willing to throw out their opinion on their own. Be open to whatever response the do give and try to turn it into a conversation.
Punish the Silent Senators:
If a particular senator simply refuses to speak up over and over again, remove them from the senate. Maybe have a referendum about it before hand and only remove them with a majority. They are not being removed due to inactivity, they are being removed because they are not doing the job. The same guidelines used when banning nobles who do not follow orders would apply.
I posted something in the Development forum about allowing more flexibility in the referendum system, but I didn't make a formal feature request because it seems like it was intentionally limited to the current options. I was going to write a full request if it was really just something that hadn't been done/considered.
Your system looks a lot like what I envisioned. I like how the Rule is stated, but the Optional portion is too strict. Requiring word on every single referendum can be either very intensive or very subdued depending on the volume of referendums. I think the allowance system is more forgiving to less active players that still want to participate. I see no issue with the Culture, though that is already in place as much as possible. It does get a little ridiculous when you have to start sending out 12 private letters. The punishment does go with what I've been saying in that the characters are essentially losing their job for lack of doing it. The thing that I don't like is that "over and over again" doesn't give any warning as to when something will happen (or if it will actually happen at all). It also leaves the door wide open for corruption, but that's an IC consideration.
