I didn't noticed this post, sorry.
What you are proposing is, if I understand correctly, is some kind of weight loader for airplanes that don't come with their own ?
What's wrong with the default loader page from FSX ? This is usually what you are supposed to use, if an airplane doesn't come with its own, and you can change both the Fuel AND the cargo load and the airplane weight and balance WILL change.
And, if you want to have more options, couldn't just use that Topcat program ?
GSX DOES read the actual weight of all stations, regardless how which method you used to set it. Either you used the default loader, or some airplane-specific one (like the one from PMDG) or some external method such as the Topcat program, as long as those method affect the airplane station load weight, GSX WILL read it from Simconnect, and will estimate the number of passengers from that.
It might not very precise but, the number of passengers is only used to figure out the time spent boarding and the number of baggage carts, which can only be 1, 2 or 3 or the type of the bus/van/minivan used so, we don't really need to be precise on that, as long as it's in the correct ballpark.