Ah, found the implementation and Martin Stjernholm's neat docs for it: http://pike.ida.liu.se/development/cvs/view.xml?module=Pike&file=7.8/lib...
CMS had an XSLT module which supported a large subset of XPath 1.0, or possibly the full standard. I wondered if this was nowadays available in Pike, and whether this was what the fuzzy reference referred to. It was neither, not both, as I first hoped.
(Parser.XML.)SimpleDOM::simple_path(string xpath, void|int xml_format) handles said small subset of XPath (which, as such, can be seen as a reduced or crippled XPath implementation for tools that need a larger or full implementation). I have been enlightened, and will return to my hibernation sequence momentarily. :-)