iRule to redirect F5 traffic based on keyword content.

First, you'll need to associate an http monitor to your pool, the monitor should check the uri /A/B/Offline.html and search for the Receive String "Online"

Then, if the user should be redireted to something.home.com:9001/Maintenance.htm, you can create and associate a http profile to your VS, in the profile you have a setting called "Fallback Host", if no member is up for that VS, then the fallback is sent to the client as the location of a 302 redirect.