Like making a server message that pops up in every server running, I have no idea, didn't found anything on how to do this, so I haven't created a script yet... I believe this is possible (I saw a message appear in roblox themepark tycoon 2 that said that they were shutting down servers in a certain amount of time) it would be very helful for a game I'm making :P
You would need an external server to do that. I have set one up with python before - a simple webserver (apache, or nginx) which hooks into python (or node) code.
That server would have a "messages_to_broadcast" table, and you'd have game instances which poll it for new messages occasionally. If one is received, that game server would send it out to all players.
So the key thing to set up is an independent spot for all servers to get information from.