[imp] How to create a 'system maintenance' popup

[Tom Leach]

I've poked around the mailing lists, FAQ and Wiki, but I can't find any 
info on what should be a fairly routine issue.
I need to find a way to send an alert (popup preferably) to everyone who 
is currently logged in to the IMP system (a subset of my IMAP users) to 
inform them on impending system maintenance (ie: a popup about the 
server going down in 5 minutes).
I don't want to send an IMAP alert since I only need to bring down the 
IMP web server and not the IMAP server.
I don't want to just send mail to the people who are currently logged in 
since they may not have new mail notifications turned on.

I'm looking for something like 'shutdown' on Unix systems that can send 
a message out to everyone currently logged in.

Thanks for any pointers.
Tom Leach
leach at coas.oregonstate.edu