Actually, his idea of a database for each user could still work, even though there is sharing of data between users. Take each database, and turn it into an executable object, which reads/writes its own data, and which communicates with other objects for sharing. It's like taking the actor model of computation, and orienting it for database use. I don't know of any working example of where this has been done, but I don't see why it wouldn't be feasible.

http://en.wikipedia.org/wiki/Actor_model