I'm not sure how hard this will be to implement, though. I didn't find an
obvious function in libxslt that would take a context/start element in
addition to the document to be transformed, so someone needs to investigate
how libxslt works here (we normally call the xsltApplyStylesheetUser()
function), and what it takes to start the transform from a different node
(e.g. if calling xsltProcessOneNode() works out-of-the-box, or if there is
a setup required beforehand).

Could you file a feature request (i.e. bug) on the bug tracker for now?