Prosody has an option for filtering MUC messages with mod_csi_battery_saver (which is the currently recommended community module for CSI), I don't think ejabberd does though.
> groupchat messages must set a subject or have the user’s username or nickname mentioned in the messages (or be encrypted) to count as “important”.
Although now that I read the spec again, it seems general enough that maybe it could be used to implement behavior like that? I don't think I've seen any servers that do, though.