Presenting an unavailable webpage when pool is down (Thick Client)

I think wnat you may be after is the LB_FAILED event

when LB_FAILED {
         Either all pool members are down OR the pool member selected has not responded
        HTTP::respond 503 content "Sorry your page is down" noserver Retry-After 300
        return
}

Alternatively if you happen to have the sorry page locally stored in an iFile;-

when LB_FAILED {
         Either all pool members are down OR the pool member selected has not responded
        HTTP::respond 503 -version 1.1 content [ifile get "sorry_page_html"] noserver Content-Type "text/html;charset=utf-8" Server $server Retry-After 300 noserver
            return

}