Nimbostratus
MVPHow in depth do you want to go with this? You can setup the iRule to monitor the number of active pool members, and if it is ZERO, it can HTTP respond with a custom web page hosted on the F5 completely --- index pages, images, css, etc. You would need to use iFiles. The following should be enough to set you on the correct path: Solution: Create a F5 Maintenance Page with Image (iRule/iFile) - A step by step procedure. | DevCentral.
Keep in mind you COULD go as fancy as a 'banner' with the backends still responding, but under maintenance. For something as complicated like that, you would need to work with your web site development team and perhaps have the maintenance banner as a normally loaded element but with a z-index that hides it, or even a visibility flag set to false. You could then have the F5 BIG-IP under a certain condition change the flag via the HTTP stream profile or something. Odds are you are looking at the first option ;)