The latest version of the Casper Suite now has something called "Maintenance Pages" under the System Settings section that you can customize with a message.
As for making that show up in Self Service, I'm not too sure, but I think you'd have to disable Tomcat while doing the work on the JSS. But there may be another way I'm not aware of or thinking of.
...And actually, I'm not clear how that page would get delivered to Self Service if Tomcat is off, so maybe I'm wrong on that last assumption. We haven't had the need to do anything like this, so I'm not too sure. It would be interesting to know however, so I'll be watching the thread for tips.
