There's no such thing as a standard "passenger number" in the simulator, like there is for the fuel. Every time you see a reference to the passenger number in any airplane, is a simulation created by the airplane code to affect the payload stations, which are in fact generic, don't have a "type" ( cargo, pax, etc. ) that could be possibly read or written by other add-on.
But even if there was such thing like a standard variable to control the passenger number, it would be very like that most of the advanced 3rd airplane products won't allow other add-on to set it, just like fuel, which is why, we don't touch the fuel quantity on PMDG or the Fenix, and the same is valid for passengers: we don't touch the plane payload, and in this case not even for default airplanes, those that GSX *does* refuel, and that's for the precise reason that payload station are not clearly defined in the sim about what they really represent.
So, it's just a visual simulation right now.
Of course, any airplane developer has the means to interact with GSX through its published variables, so they can synchronize their own internal passenger simulation with GSX, something that was made very effectively by some developers in P3D, but the same features are possible in MSFS as well, it's just a matter of the airplane developers willingness to integrate with GSX.
