Question : Page redirection to Maintenance page when pool members are down

there are a couple of options such as what bigip version are you running (9.x/10.x or 11.x)?, where is maintenance page (server or bigip)?, would you like to change url shown on a browser?

if maintenance page is on server and you just want http redirection, Vitaliy's irule or something like below may be usable.

e.g.

root@(ve11a)(cfg-sync Changes Pending)(Active)(/Common)(tmos) list ltm rule myrule
ltm rule myrule {
    when HTTP_REQUEST {
  if { [active_members [LB::server pool]] < 1 } {
    HTTP::redirect "http://www.google.com"
  }
}
}

hope this helps.