psgml and XInclude elements in DocBook XML

Does anyone know of a way to make psgml ignore, or allow <xi:include>
elements in <programlisting>?

Since xsltproc is a lot faster than the Java XSLT processors, I would
like to use XInclude elements to include text files into
<programlisting> elements, as shown in Bob Stayton's book:
  http://www.sagehill.net/xml/docbookxsl/SolveProblems.html#ProgramListings

The problem is that <xi:include> is not a legal element at that point,
according to the DocBook XML 4.2 DTD.  Is there a non-intrusive way of
adding this element to the DTD in the appropriate way?  Ie. a way that
doesn't involve changing the DTD?

Are there modifications that can be done to psgml's understanding of
the DocBook XML DTD?

Thanx!


- Steinar