fbx is a file format. Anyone can modify any file contents as they see fit, so no there are no legal barriers what so ever to begin with.
On top of it Autodesk released all of the definitions and base code to manage/handle the format as open source, so technically its entirely possible to re-inplement the 2018 format exactly as its ment to be directly into blender.
The challenge is figuring out what goes where from the way blender builds things to the way the FBX builds things - and it’s quite a challenge because you can have so many Many different types of things.
lights, meshes, materials, armatures, etc. And that’s basically scratching the surface of what one needs to handle.
Also, selecting individual bones for export shouldn’t have any effect.
It’s selected Objects and a bone isn’t an Object in blender - ergo it should (because I dont remember if it actually does or not) not be taken into consideration and just export the whole armor object.