Catalog resolution?

I'm currently using expat to parse DITA documents by
using XML_SetExternalEntityRefHandler to look at the
system identifier, create a reference to a local dir, 
and parse it using XML_ExternalEntityParserCreate.
(Shorthand version; I can post the code if necessary.)

Now we need to support specializations, some of which
use XML Catalogs.  I don't see a catalog resolver in
expat; am I missing something?  Any ideas on how to
add catalog resolution without using Java or any GPL
software?

Thanks!

-- Jeremy H. Griffith, at Omni Systems Inc.
  <[email protected]>  http://www.omsys.com/