FAILED selecting pool based on DNS reverse lookup result.

another solution off top of my head is to trigger LB_FAILED and reselect pool.

e.g.

 config

root@ve10(Active)(tmos) list ltm rule qux
ltm rule qux {
    when CLIENT_ACCEPTED {
  log local0. ""
   8.8.8.8 is dns server
   203.216.231.189 is yahoo.co.jp
  NAME::lookup @8.8.8.8 inet -ptr 203.216.231.189
   fake node (to trigger LB_FAILED)
  node 1.2.3.4 80
}
when NAME_RESOLVED {
  log local0. ""
  if { [domain [NAME::response] 1] eq "jp" } {
    set pool_name foo
  } else {
    set pool_name default
  }
}
when LB_FAILED {
  log local0. ""
  LB::reselect pool $pool_name
}
when SERVER_CONNECTED {
  log local0. ""
}
}

 log

[root@ve10:Active] config  tail -f /var/log/ltm
Apr 16 17:44:53 local/tmm info tmm[4955]: Rule qux :
Apr 16 17:44:53 local/tmm info tmm[4955]: Rule qux :
Apr 16 17:45:05 local/tmm info tmm[4955]: Rule qux :
Apr 16 17:45:05 local/tmm info tmm[4955]: Rule qux :