Irule bassed on html

I think I misunderstood what you were trying to do. I think you could do something like what you've described using an iRule which checks if the virtual server's default pool has no active members and selects a secondary pool in that case.

when CLIENT_ACCEPTED {

    Save the name of the virtual server's default pool 
    before it is potentially changed by this iRule
   set default_pool [LB::server pool]
}
when HTTP_REQUEST {

    Check if the virtual server pool is down
   if {[active_members $default_pool] == 0 }{

       Select an alternate pool
      pool second_pool
   }
}

Aaron