Matt is correct; we did at one time have a MOTD zimlet that also doubled as an emergency notification system. It was retired quite a while ago, and the code isn't public (it's really, really old, and really, really bad -- I'm pretty sure it was my first zimlet, and it's doing really stupid things like synchronous ajax calls, because hey, async is haaaaard), but it was fairly straightforward. We had a simple backend store of messages (primairly tips & tricks, like the annoying ones in desktop software). When the zimlet loaded, it first checked for any messages classed as "emergency" and displayed that in a modal dialog window (basically, one of these:
https://wiki.zimbra.com/wiki/ZCS_6.0:Zimlet_Developers_Guide:Examples:Simple_Dialog). If no emergency messages are found, then it gets a random message and displays that (again, this was meant to show helpful tips, not a true MOTD, hence the randomness). There was a "don't show at login" checkbox (which was ignored for emergency messages).