UPDATE: Miriam at Bloghead comments on the Philologos essay:
All very brilliant. But his theory has one weakness: why would a scribe need a box in which he can write, in dust, words and sums that can then be erased? It's hard to imagine.
Perhaps to scribble draft notes and sums before using expensive writing materials to write up the final draft? That's not far from what she goes on to suggest, but scribes would presumably find this useful too.
Whether or not writing in a box of sand would be halakhically acceptable on the sabbath, as Philologos seems to suggest, I leave to those who know more about such things than I do.