Stuart,
To my knowledge, the alternator wouldn't charge unless the #1 terminal was supplied with a initial charge. This is true unless the regulator has been changed to one suitable for single wire operation.
There is a regulator suitable for single wire service that looks identical to the three wire version. It will allow the alternator to charge with only the BATT wire connected, with the sensing wire connected or with all three wires connected.
If this is accurate, I will surmise this is what you have. As far as the diode goes, there is a after market dongle for the DA plug that is used when there is no idiot light in the circuit.
This plug has a small diode installed in series with the #1 terminal and the vehicle power source. It's function is prevent engine run-on when the excitation voltage is pulled from somewhere not switched from the ignition system
Not being a electronics type, I would guess that if this small diode would work, then a small diode in a light emitting configuration would work too. I could be all wet on this though.
Stuart
