I think it will need a code ass that can only be used when on uses the platform, then when a recorder of some kind is used, the video go with the watermark, when is legit, it is hidden from the screen, but is there, so there is no way around it, but to have the code, soi can think how that ca work using the blockchain.
Is something like this even possible? And what about someone is using the platform legally but make a video capture of the fullscreen video. In this case it would produce a new video file from a screen capture.
No way you can be protected from this, but in case somebody will really decide to record from another screen, the picture is probably no good and sound is even worse. Viewers nowadays are quite picky.