The Asciidoctor-to-XML and XML-to-Output classes are separate, so you can invoke just the latter without the former. Of course, you will need to make sure that the IsoDoc XML you are passing to the generators is valid.
IsoDoc::…::WordConvert are initialised in the existing gems with a hash giving the fonts to be used in the document (to be injected in the document SCSS stylesheets), the script of the document (to be used to pick the right font, in case of default font settings), and the
i18nyaml YAML file for localization. All existing gems have defaults set for these values on the Asciidoctor side invoking the class, so all parameters are optional.
Once you have the classes set up, all you need to do is invoke the conversion of XML to the target format, with the method
.convert(filename, xml), where XML is the IsoDoc XML.