For anyone interested in this problem, check out this post, which led me to:
<foreach param="dir" absparam="absdir" target="minify.directory">
<fileset dir="${publicdir}/css">
<type type="dir" />
<depth max="0" min="0" />
</fileset>
</foreach>
This allows me to specify a directory constraint when iterating, thus passing through the relative and absolute directory name as opposed to the filename.