Displaying maintenance page from the F5 using ifile and irule

I've done something similar using the LB_FAILED event to get the same results. In this example there are three iFiles (CSS, PNG, HTML). You can use an iRule like this one:

when LB_FAILED {
   switch [HTTP::path] {
     "/css/Unavailable.css" {
       HTTP::respond 200 content [ifile get "Maintenance-css"] "Content-Type" "text/css"
       return
     }
     "/img/MaintenanceLogo.png" {
       HTTP::respond 200 content [ifile get "Maintenance-png"] "Content-Type" "image/png"
       return
     }
   }
   HTTP::respond 200 content [ifile get "Maintenance-html"] "Content-Type" "text/html"
}

Hope this example helps!