[gsnedders]

AFAIK:

The only case that trumps a red traffic light is when given a signal by an authorised person (e.g., police officers, traffic officers, etc).

I think under a literal interpretation of the law you are obliged to commit an offence if you are beckoned on across a stop line at a red traffic light; you can either refuse the instruction to be beckoned on (an offence) or you can cross the stop line (an offence). That said, there's plenty of habit of the beckoning taking precedent over the lights.

Basically the only time you see any police officer instructing traffic from a vehicle is when on a motorbike, typically when they're part of an escort.