Thanks so much for that - it certainly narrows down the pins I need to try.
That long-pinned header would presumably mean that I'd be accessing the spare pins on the LED-side of the Scroll pHAT. I'd rather leave that side clear to keep the display unobstructed and for possible fitting of a diffuser when it becomes available, so what I might do is just attach the standard header to both the Pi Zero and the pHAT, and test for (and then leave connected) the required pins by plugging in separate wires.