It would be interesting to know what the shadow would fall on or point to at winter solstice. If there is something close by in the right direction then, yeah, it's posible. At Knowth there is a tall thin standing stone outside the entrance to one of the tombs that casts it's shadow onto a line carved into the door stone at sunrise on Winter Solstace. This was to allow those standing outside to 'see the moment'. Obviously a big, big stick atop Silbury would allow a lot of people to 'see the moment'.