[xml] post-XInclude validation using schema, not DTD

[Ben Liblit <liblit cs wisc edu>]

If I run xmllint with "xmllint --noxincludenode --postvalid ...", then 
xmllint will check my document against a DTD after all XInclude 
inclusions have been processed.

How do I do post-XInclude validation using a W3C XML Schema instead? 
The obvious "xmllint --noxincludenode --postvalid --schema foo.xsd ..." 
does not work; xmllint still tries to load a DTD.  Inspection of the 
"xmllint.c" source code confirms that "--postvalid" turns on DTD loading:

        } else if ((!strcmp(argv[i], "-postvalid")) ||
                 (!strcmp(argv[i], "--postvalid"))) {
            postvalid++;
            loaddtd++;
            options |= XML_PARSE_DTDLOAD;

Is there simply no way to postvalidate using RelaxNG or W3C XML Schemas?