Actually I think they do give us a clue. Your point (unless I’ve misunderstood it) was that, “It could work at some of the medium sized henges but if there was anything to be seen at Avebury they would have needed giant screens to relay it as it's 200 m from bank to the centre...” In actual fact it’s demonstrably the case that spectators on the south bank of the south-east quadrant are able to see clearly any activities going on in or around the Cove because the line of sight is probably as good from there today as it was in the Neolithic (so too presumably are the acoustics). Given that the Bank was even higher in the Neolithic it’s probably not unreasonable to assume that both would have been even better at that time.

All’s the world a stage (except at Avebury were there were probably at least four, “That ends this strange eventful history... Sans teeth, sans eyes, sans taste, sans everything” ;-)