Catching up on a pile of "Current Archaeology" I am up to 132. This has an article on the excavation of a long mound on Crickley Hill, which is [?was] thought to be an "offerings mound". This, like Silbury, was built up over time by 'offerings' - in its case of soul rather than chalk. Offerings continued until at least Roman times (mention is also made of a 17thC mound), when the practice was to punch a long hole down and drop the likes of coins in. In a massive earthwork like Silbury it would be little wonder that no finds have been [IIRC] found. So, in my estimation, either an offerings mound or a viewing platform are the neatest solutions to its being so 'clean'