I have no idea what pfpx or simbrief are but, assuming they are some kind of planning utilities, on the last pages of the GSX manual, there are some indication for software developers how to interface with GSX, and many products already use them, for the precise reason of telling GSX the number of passenger *they* have calculated on their own, and have the same number in GSX.
I don't know how these products works but, if they have a way to set L: variables in the sim (so they should have a Gauge or a Module, but it's also possible with FSUIPC+LUA or Linda), they could just add the same features several airplane developers have added to their planes to communicate with GSX.
