I've now managed to get a CODEREF to the routine I was after like this:

    eval_pv(
        "use DynaLoader;" \
        "use XML::LibXML;" \
        "my $pnts_symref = DynaLoader::dl_find_symbol_anywhere('PmmNod
+eToSv');" \
        "my $coderef = DynaLoader::dl_install_xsub('kousei::PmmNodeToS
+v', $pnts_symref);"
        TRUE
    );
[download]

Trouble is, now I can't find how to call it from C with the args I need to pass. Calling it from perl is fine via &$coderef(), but the xmlNode I want to pass to it is in C.
Any clues?