Colin,
Being anything but knowledgeable about electrical stuff, I hesitate to give advice on anything I haven't tried myself. Here are some observations, though:
a) You are aware, are you not, that the door lock relay is only to change the action of the headlight flasher from contact-to-ground into current-to-something-else and is not needed if you use a momentary switch to give a power jab to the OD relay to stir it into on and off action?
b) As I understand it, the 4th gear switch completes a circuit from terminal 85 on the OD relay to ground. This makes the relay send power to the OD solenoid that keeps the OD working as long as power goes to it. So, if you can rig something from your back-up light switch, which is a switch to ground, or the back-up light relay that cuts the power from the OD relay to the solenoid or between the fusebox and terminal 15 on the OD relay when you engage reverse, that should do the trick. But as I said, I'm out of my depth here, and I'd much rather have you follow someone else's advice.
Good Luck,
Bob S.