Have you seen the Captain's Challenge in operation in the Holden Cup? They've had it for 2 years and there's still no push for it to come into the NRL. If we had to do it, I would restrict it to tries only and scrap the video ref, except for the challenges.
I haven't seen it no, it was something I'd seen in tennis that seemed to work well. The restrictions you mentioned were exactly what I was thinking, take the whole process (and the associated pressure of over/under use) away from the ref and let him get on with refereeing the game.