Ah, of course I figured it out myself right after posting this. A trivial problem.
When the code was a standalone module, I was waiting for the SimStart system event before I called SimConnect_RequestDataOnSimObject(). But for a panel module, no SimStart or SimStop events are generated. Which, I guess, makes sense as the panel module is loaded only when the aircraft is flying and the simulation is running.
When I made that logic simpler, it again works as expected. Great. I’ll keep this thread here, anyway, in case it helps somebody else later.
(See GitHub - tml1024/flying-brick: Sample aircraft for MSFS built from scratch )