using xpath, not that I know of...
You could write a small stylesheet, transform the xml into its sorted order, and then select the nodes in this order.
<xsl:apply-templates select="//nodes">
<xsl:sort select="wibble" order="ascending"/>
</xsl:apply-templates>
or
<xsl:for-each select="womble">
<xsl:sort select="@hatsize" order="descending"/>
...
</xsl:for-each>