Hey,
while working on our current tool i was looking for a way to get the airport data (BGL information) for the Airports that are in the facility list that is provided by the SimConnect-SDK.
At the moment it seems only possbile to read all the BGL files at startup and store them, and then merging the airports from the facility List with them each time a new airport is added to the facility list.
I do not know how FSDT is doing it with theier GSX Addon, but it seems like have found a way to get them.