Currently in 4.0(4) you can use public distribution lists for this but there's no way to delegate this right to just a few folks for specific lists (you can limit subscribers from sending messages to _any_ DL at all, but not for just specific lists).
In the upcoming 4.0(5) release there will also be broadcast messaging capabilities (seperate from sending messages to DLs) which can be turned on/off for users in the Class of Service they are assigned to. Broadcast messages are sent to all users on a Unity server and you have the option of forcing users to listen to them ahead of all other messages and not being able to skip through them and the like.
