The uppercase version of key="Edition" is used in this context:
<xsl:template match="edition" mode="titlepage.mode">
<xsl:apply-templates mode="titlepage.mode"/>
<xsl:call-template name="gentext.space"/>
<xsl:call-template name="gentext">
<xsl:with-param name="key" select="'Edition'"/>
</xsl:call-template>
</xsl:template>
If you customize that l:gentext to be lowercase, it should work.
Bob Stayton
Sagehill Enterprises
DocBook Consulting
bobs@sagehill.net