The problem is not broadcasting - it's having a client listening to the broadcasts.
What client is connected to jboss? If it's a web browser then the only thing i can suggest is an "included jsp" that checks for a shutdown message. when a page is displayed.
This will work only if the user refreshes the page. Otherwise you could maybe use something like sidebar to continously check.
If you have fat clients then if becomes a lot easier - eg with JMS.
You could always sms/email the shutdown notification.