for-each-group grouping accented versions of letters

So I've got an XML index file, which is too large for some downstream
processing to be entirely pleased with.  The requirement is to split the
file up, grouping index entries (index-0 elements; the index element is
the overall container element) by the first character of their child
heading element.

Using XSLT 2.0, this is pretty easy:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet exclude-result-prefixes="xs xd" version="2.0"
  xmlns:xd="http://www.oxygenxml.com/ns/doc/xsl"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:template match="/wkna-shared-cms/index">
    <xsl:for-each-group group-by="substring(heading,1,1)" select="index-0">
      <xsl:sort select="./heading"/>
      <xsl:result-document href="eitaindex+Topical_Index_{current-grouping-key()}.xml">
        <wkna-shared-cms>
          <index area="{/wkna-shared-cms/index/@area}"
            xml:lang="{/wkna-shared-cms/index/@xml:lang}">
            <num cite="Topical Index {current-grouping-key()}">
              <xsl:sequence select="current-grouping-key()"/>
            </num>
            <xsl:copy-of select="/wkna-shared-cms/index/index-metadata"/>
            <xsl:copy-of select="current-group()"/>
          </index>
        </wkna-shared-cms>
      </xsl:result-document>
    </xsl:for-each-group>
  </xsl:template>
</xsl:stylesheet>

The problem is that some of the initial characters of the headings have
accents, and it's desired that the accented characters and the
unaccented characters group together, so that E and I and J, etc. all
group together in a group with a current-grouping-key() of "E".

I can imagine doing this in a painful way with conditional statements
and an exhaustive list of characters, but I'm hoping someone can tell me
there's a better way.

Thanks!

-- Graydon