What is an "xml-stylesheet reference" exactly?

If you mean a processing that attaches an xsl stylesheet (eg <?xml-stylesheet type= "text/xls" href="xls_style.xlss"?>, then it's a processing instruction, and can be inserted as a regular PI: create an element with a tag of '#PI' and set its target to xml-stylesheet (using set_target) and its data to type= "text/xls" href="xls_style.xlss" (using set_data).

Does this help?