The work to match interfaces with DOM will tend to drive this decision, likely a single simple solution will be possible. Suggest adding X3D to W3C browser Amaya. These tests are being placed online. Many details on this topic are described in the X3D Retreat minutes on unified field model . It would be helpful if anyone can come with counterexamples that won't work under this approach.
Issues seem to be social-contract related, rather than technical.
Separate technical problem is where URN's are stored on a particular platform. Web3D Universal Media group researched the problem and chose a good local storage solution for each major operating-system platform. Modifying the Windows registry was flagged as a potentially hazardous practice, however. An open question in X3D is how to represent multifield URLs - attributes are geared to individual strings, so use of multiple URL tags may be necessary. Jamal Mark Rudolph was unable to attend, unfortunately.