"The real problem is the precedent it creates, and not just at Silbury. Anyone with a media-friendly suggestion will have to be taken seriously in the wake of this foreign object being placed where it shouldn't. "
But it doesn't have to be media friendly or involve asking anyone to take the idea seriously. There are loads and loads and loads of people who want to leave all manner of stuff (including paintings!) and bury crystals and all the rest of it at scheduled sites as part of THEIR perceived and self-proclaimed right to interact with places as they see fit. Who in EH will be able to look them in the eye and say don't?