The Schemas WG did leave the door open to supplying a definition of how to refer to public IDs in the schema location even though they're not interested in doing it themselves.
Some organizations use public IDs.
We should ask the Schemas WG to either define a public ID for their DTDs and schemas, or say they won't. The idea of the catalog is to pass in an external ID and get back a s.o.i. We would need to define what happens if the external ID is a system or public ID, and the knowledge of which is which. The other pov is that we can justify a special case for URNs. If the catalog processor is told that something is a system ID when it's passed in, then it determines some of what to do based on that.