"Obviously, with sets, the location can be described in several ways, so it is not a Unique Resource Locatior - only RL."

Wouldn't it be possible to implement the filesystem with tags and then specify an ordering of the tags to map the graph into the usual hierarchy? The first part of the URI would specify the mapping (maybe just a name for a particular mapping), and the second would traverse the hierarchical view in the usual way.

This way, legacy apps could still think in terms of a tree while exposing a more powerful structure to new apps.