Thanks @DrXibber for your insights. Do you think it'd be possible to use one of these custom properties to have the app pass around NSData* representing a frame that ultimately gets shown by the Camera Extension? That way we wouldn't need complicated XPC (which nobody seems to have figured out yet except perhaps @ssmith_c in https://developer.apple.com/forums/thread/706184 ?) or even use another sink stream (also suggested by @ssmith_c).

@smith_c could you post sample code for that method? I've added a second device with a sink stream. But then I'm stuck on the app side: accessing the CMIODevice that corresponds to my camera and sending frames to its sink stream is not trivial (CMIO is C only API, doing this in Swift isn't easy).