We use the DocBook XML and XSL distribution archives without any changes to them. The reference files are located in the src/zip folder and are expanded into the target/ directory before the rendering process.

The file names must be reflected in the docbook.xml.version and docbook.xsl.version properties in the docbook.properties file. If you want to use e.g. a newer XSL version, you can put it into src/zip and update the docbook.properties to reflect this change. Let us know how it works out for you.