One of the two throttle servo settings (normal or reversed) on the transmitter should make the ESC do what you want. I don't see why it would matter if the ESC is on or off when you flip the throttle servo reverse switch; as long as you have the throttle trim in the neutral setting.
Just in case you don't have the manual...
EXT Manual:
http://manuals.hobbico.com/dtx/dtxd29-manual.pdf