Unity transforms have the hasChanged property you could use to detect if a transform is changed and run a frame counter to generate a file you later can process. But I guess this is very complicated as VR Rigs have multiple transform targets you must grab every frame to get your “video capture” of the play session.

After that you should get the object data in edit mode and generate your FBX based on the mesh and transform targets. Unity can't create FBX from plain data so you have to use the FBX SDK or any equivalent solution as Unity Plugin