I don’t know of a way to modify the HTTP headers that are coming from the THETA V built-in server for motionJPEG.
the underlying OS in the camera is Android. You can write a plug-in for the THETA V/Z1 that streams higher-quality video (like the RTSP plug-in) and have the relay basically be inside the camera.
It seems like you’ve researched this quite a bit.
The RTSP plug-in and the Wireless Streaming Plug-in are both open source. You can see what they did and try to modify it.
I’m assuming that the reverse proxy solution described below, won’t work or is not practical?