LB::server pool

Yeah, I'm not sure whether LB::server pool would return the VS default pool in LB_FAILED. If it doesn't, you could save the name of the pool in CLIENT_ACCEPTED and then reference that variable in LB_FAILED. Similarly, I don't know if HTTP::host would be available then either.

when CLIENT_ACCEPTED {
   set default_pool [LB::server pool]
}
when HTTP_REQUEST {
   if {[HTTP::host] eq ""}{
      set host [IP::local_addr]
   } else {
      set host [HTTP::host]
   }
}
when LB_FAILED {
   if { [active_members $default_pool] < 1 } {
      HTTP::fallback "http://sorry-$host"
   }
}

Aaron