Can I suggest this functionality is restricted to on the ground with all engines off?
That's exactly how it is now. Generally speaking, everything in GSX requires to be on ground with engines off, with the only exception of allowing to start engines while pushing, but at that time you are already leaving the parking spot. When you are parked, you cannot call any service if engines are running and parking brakes (or wheel chocks) are not set.
In addition to that general rule, we don't even constantly check for refueling.
The only time we start monitoring changes in the fuel levels, is after the user has called the GSX Fuel service, and after the fuel hose is attached, and the "please use the refueling system of your aircraft to refuel" (on a plane configured as a custom fuel system) will appear. Everything happening to the fuel before that, we just don't care.
Also - did you have any more thoughts on allowing us to read when GSX requests a door on the aircraft to be open/closed, so that we could action these requests automatically inside our aircraft?
I'll think about it, we already have some published variables to let 3rd party apps knowing the GSX service status, we might add the waiting status too.