How do I configure a simple TCP proxy that can rewrite SQL from a client to a MySQL server?

Ok, I was able to get it working using the information provided here. First I had to create a Standard Virtual Server with a TCP protocol and profile, with Source Address Translation set to Auto Map. Then I added a pool with the MySQL server in it and gave it a TCP monitor and attached them together with the Virtual Server.

Next comes the iRule, which is below and is correctly adding an index hint to the SQL "on the wire." The code is still a bit ugly with magic numbers and strings, but it works:

when CLIENT_ACCEPTED {
  TCP::release
  TCP::collect
}

when CLIENT_DATA {
  binary scan [TCP::payload] ica* length cmd sql
  set cmd [expr { $cmd & 0xff }]
  if { $cmd eq 3 } {
    set length [expr { $length & 0xffffffff }]
    if { $sql contains "WHERE Contact.IsUser <=> '0' AND `ContactGroupAssign`.`DateCreated`" } {
      set location [string first "FROM ContactGroupAssign INNER" $sql]
      if { $location > 5 } {
        TCP::payload replace [expr $location + 29] 0 "use index (DateCreated) "
        set newLen [expr $length + 24]
        set replacement [binary format i $newLen]
        TCP::payload replace 0 4 $replacement
        log local0.alert "MySQL rewrite: added \"use index (DateCreated) \""
      }
   }
  }
  TCP::release
  TCP::collect
}