After some tinkering around with Mcrcon i finally got my broadcast message script to work. Sadly i discovered that the Broadcasted message to people on the server shows up in a box that takes control away form the player and you need to push ok before you get control back!
This is not great, since it is way to disruptive to the player! I would strongly suggest that you make a banner or something that does not take away control and only stays for 30 sec. Like a banner that roles down and then disappears after 20 to 30 sec.
It would also be nice if you could send a message to the global chat with a chat command from a script