Had my 85 C4 Doug Nash 4+3 transmission rebuilt, but there remains an issue with the overdrive switch that isn't supplying 12v to the solenoid that enable the +3 overdrive, and apparently affects reverse according to the Transmission shop...

Does anyone have suggestions or a detailed schematic for this?

The O/D section of the Nash unit has nothing to do with reverse not working. The O/D clutches either hold, or they do not. A problem in reverse would also translate to a problem in the forward gears. I can't remember off the top of my head if the O/D solenoid is GROUNDED by the switch, or if the switch provides power to the solenoid. MOST GM systems have 12v power to the solenoids any time the key is on, and the ECM or switch provides a ground. I can look at the diagrams and post how it works later. What kind of issue are you having with reverse?

A quick glance at the diagrams shows the O/D switch only provides a ground signal to the ECM. From there, the ECM sends out a signal to the O/D relay, which then sends a signal to the O/D solenoid in the trans.