You could embed a script on each site at any time. Have the script check a URL on your server, it can be simply a text file with a 0 or 1, or you can put a message in there (JSON is a good protocol to use). Have the script, client side, check a URL for that variable. If it is 1, display the message, if it is 0 do not. You might even want to encode the message in that text file in case you need to display some message to all of your website users about a delay or any other information.
You can get as fancy as you want with it, but in my head this is the best way to do this for a number of sites. Of course, if you have a lot of hits at any one time, that is a lot of clients checking that URL. It just depends on your particular situation. You could also do it server-side with some caching.