Plugin to notify users?

Hello there,

in the last months we had a bit trouble to notify users about incoming shutdowns/maintenance work (which can take a day, because our main installation is so big). We have several thousands of users, therefor it is a bit tricky to send all of them an email. Plus, many of them aren’t active currently, so they don’t need the notification.
Therefor I would like to write a plugin to show a message on the backend pages just like the PKP upgrade warning. It should work sidewide, but it shouldn’t show in the plugin gallery of each journal. Is this possible to do?

Kind regards