<!-- get the location of variable_keywords.ditamap -->
<xsl:param name="PATH2PROJ">
<xsl:apply-templates select="/processing-instruction('path2project-uri')[1]" mode="get-path2project"/>
</xsl:param>
<xsl:param name="WORKDIR">
<xsl:apply-templates select="/processing-instruction('workdir-uri')[1]" mode="get-work-dir"/>
</xsl:param>
<xsl:variable name="VAR-FILE" select="concat($WORKDIR,$PATH2PROJ,'variable_keywords.ditamap')"/>
Then use the doc() function to point to this external file:
<xsl:comment>
<xsl:value-of select="doc($VAR-FILE)/map/keydef[@keys='var_release_version']/topicmeta/keywords/keyword"/>
</xsl:comment>
If anyone knows a more elegant solution, feel free to post it back.