Just clicked on this one from links in an ancient abc/lilypond thread that had been refreshed. Anyway, to try to give some more recent information.

I did have some form of very brief revisit on this subject at some point last year when I was playing with an abc converter.

Possibly the best solution are the python programs abc2xml.py and xml2abc.py programs available from https://wim.vree.org/svgParse/index.html.   

timepart.xsl and parttime.xsl can be used to switch between the two musicxml formats when needed but I don’t think they can be used directly. This was the php routine used in the folkinfo converter

          public function xmlxlate($xsldoc){
                  $xml = new DOMDocument;
                  $xml->loadXML($this->xml);
                  $xsl = new DOMDocument;
                  $xsl->load($xsldoc);
                  $proc = new XSLTProcessor;
                  $proc->importStyleSheet($xsl);
                  return $proc->transformToXML($xml);
          }
  

The abc converter at mandolintab.net can handle all of this (eg.should detect musicxml as input) but I think uses older/different programs and I think the musicxml import program at the time it started to be hosted there was one written by the author of EasyAbc.