Well, it's certainly not ethical from a conservation point of view. I'm not an engineer but I would have thought that supports made of chalk blocks, positioned vertically and adjacent to any steel supports that are difficult or dangerous to move, would be one way of doing it. The chalk blocks would at least be of very similar material to the Silbury structure itself. Now that English Heritage have imposed what is, effectively, an information ban on their work at Silbury we may never know - unless there is someone out there willing and able to provide us with the information.