Having worked with SGML/XML for some time now, I still encounter software that does not support XML Catalogs (including software I wrote), but do support TR9401 Catalogs.
Instead of trying to update/modify older software, I found it easy to write an XSLT to convert an XML Catalog into a TR9401 catalog. The XSLT is viewable/downloadable from the following link: xmlcat-to-opencat.xsl
For features supported in XML Catalogs but not in TR9401, the XSLT will print out warnings, but continue processing.
Unsure if anyone else may find the transform useful, but it comes in handy when I need to use my old DTD parsing tools.