Re [xml-dev] 'is-a' Relationships in XML?

Apologies for harping back on the
 former thread.

Given then that I need to define the
semantics of a markup outside of that 
markup and outside of a schema,
perhaps using RDF or the like, I'd probably 
want to associate many classes and properties
or expressions with XPaths in my markup. 
If I used RDF, it seems to like URIs or URLs 
or the like (there seem to be many such 
identifier standards of late). Would there 
be a standard way to add something like 
an XPath to perhaps the end of such a URL, 
so I can point a semantic expression like 
class or property to a node in the markup? 
XQuery? It would be nice to have a semantic 
query like SPARQL be able to resolve
 it.

Thanks

Stephen D Green