Pete, are you sure the red/white turns on when the ignition is switched on (but engine is not running)?
See the diagram below for MY76. It shows that the red/white is tied together with the black at FP relay pin 30. And at the other end, it is at the 14-pin connector in the engine bay.
Hence it should follow the FP on/off cycles and could indeed be used to power the O2 sensor heater. Its gauge is only 0.5mm2, but that should be enough, since the WUR is no longer there.
