Hi!
You can export all cameras including embedded ones in OPK format using following code in built in Python console:
path = Metashape.app.getSaveFileName()
camera_keys = list(map(lambda cam: cam.key, Metashape.app.document.chunk.cameras))
Metashape.app.document.chunk.exportCameras(path, Metashape.CamerasFormatOPK, cameras=camera_keys)
Regards, Egor