Impossible to override XSLT documents used in the PDF export process

The documentation at http://platform.xwiki.org/xwiki/bin/view/AdminGuide/Configuration#HCustomizingthePDFexportLook26Feel doesn't actually work when it comes to overriding the xhtml2fo.xsl and fop.xsl stylesheets.

Although the implementation suggests that the xhtmlxsl and fopxsl properties of the XWiki.PDFClass class is supposed to contain the actual content of the xslt file, it is actually used as the name of a filesystem resource.