Need to change the HTTP Status code.

If I understand you correctly, you're trying to sometimes alter the HTTP status code of a response passing through the BIG-IP. You could just do this at the TCP level and bypass all of the messing about at the HTTP layer altogether. Something like (totally untested):

 

 

when SERVER_CONNECTED {
   if { $should_alter_response_status } {
     TCP::collect 12
  }
}
when SERVER_DATA {
   set payload [TCP::payload 12]
   regsub 200 $payload 503 payload
   TCP::payload replace 0 12 $payload
   TCP::release
}