A method to send administrative messages to active users

It would be nice if you could somehow send a message (a noticable
message, not to the users’ inboxes) to the active users so that they
can be notified that, for example, the system is going down for
maintenance at a specified time. This would smooth out the process of
doing all of these upgrades by helping to eliminate the need for admins
to contact each user individually, or just kick them out of the system
and risk lost work.