We recently released RT::Extension::Announce which gives you an easy way to insert announcements on your RT homepage so all users can see the message. You may want to display a banner during maintenance or maybe an unscheduled outage to make sure the people fielding customer tickets know that something is going on.
The messages are set and managed in a dedicated queue, created when you install the module. This allows you to manage who can post announcements using permissions on the queue. You can also show messages only to select groups if you don't need to notify everyone.