Personally, I don't see the logic in being able to do that. The game purposely limits communications to specific limits.
Besides, imagine being at war, and the invaders all sending realm-wide messages to the enemy realm, with there being nothing that the target realm can do about it. You can't ban them, or give them bad marks, or fine them, or do anything at all to get them to stop, even if the entire realm wants them to. And don't forget all the arguments that would ensue, as every responds to the OP, but none of those messages actually reach the sender, because he's not part of the realm.
Besides the fact that the mechanics involved would be fiendishly complex or non-intuitive, this would be way too easy to abuse.
