Best Practice - Maintenance Pages when LB_Failed

If you want to reference the default pool that is associated with the irule on a VS, then you can just use [LB::server pool]

when HTTP_REQUEST {
   if {[active_members [LB::server pool]] < 1 } {
      HTTP::respond 200 content
   }
}

If you want to send the connection to an alternate pool when the default pool is down, but not use a pool name, you can use an HTTP::redirect and just manage the maintenance pool via it's own VS and Dns entry. You could also setup a global static variable that all irules can access on an LTM system with a value of the alternate pool

Hope this helps