Complex group-by with saxon:evaluate?

I currently have the following section in my XSL 2.0:
   <xsl:for-each-group select="bibtex:entry"
group-by="tokenize(normalize-space(*/bibtex:refgroup),' *; *')">
    <xsl:sort select="current-grouping-key()" order="ascending"/>

This works as expected.
Now, I've tried to replace bibtex:refgroup with a stylesheet parameter
(say $groupby). I know this does not work in XSLT without extensions.
However I could not even
get it to work using saxon:evaluate (in Saxon 8.6), though - as far as I
understand - it should be possible. Maybe someone could help me out?

Moritz Ringler (xsl-list@xxxxxxxxxxxxxxxxx)