First, I used a Recipe Manager to get all recipes. I grouped them by "Get Produced In" which gives me a set of ~8 buildables (FGBuildable Class Reference). I then used the Recipe Manager's "Find Building Descriptor by Class", but it only returns Building Descriptors (FGBuildingDescriptor Class Reference) for constructor and assembler because those are the only buildings I can actually build in the game (things like the Manufacturer are not unlocked yet).
However, in my mod I would like to show icons of all available machines even if they're not unlocked yet. Do you have any idea how to achieve that?