iRule To Control Access Based on Source and Destination Addresses

You seem to have a good grasp, however I don't think I was paying enough attention when I made my other update. I only mentioned /Common/dg_ftp_out as you had referenced it but not defined it. I don't really see that it's necessary - you could get away with what's below instead;-

when CLIENT ACCEPTED {
   if {!([class match [IP::client_addr] equals dg_allowed_ftp_sources] && [class match [IP::local_addr] equals dg_allowed_ftp_destinations])} { 
      discard
      return
   }
}