Providing advance notice before maintenance without node refresh?

So, I have an Intranet / collaborative site that has a lot of users logged in at any given time day or night. Inevitably when i need to do maintenance, someone will have to get kicked off / lose their session. Since many are writing content, I'd like to give them 10 minutes notice or whatever. My poor man's solution is to have a sticky message that posts on all nodes, but if someone doesn't refresh their node.........well......they are screwed. Is there a module using jQuery or something that when activated through the admin section will pop up a message or change a graphic? I can probably hack this myself, but my skills are lacking, so I thought I'd check if there is an existing elegant solution.