Although it sound like doable I have considerations of this working...
When playing WHDLoad games the RTG screen goes black and the native AGA screen that was Grey/White plays the game...
Well... running the Mediator TVTuner Program with the Composite Input theoretically might work ONLY in full screen IF it overlays the black RTG screen.
I haven't tried it despite I have an TVTuner in my Mediator TX so I don't know if it can be done but you can try it.
Apart from that, I have the impression that the quality would kind suck imho.