Retrieving custom doc properties in Preview XSLT stylesheets

I'm trying to get custom topic properties in the Preview XSL code.  I tried following the examples found in infoshare.params.xsl and have no problem retrieving out-of-the-box doc properties like DOC-LANGUAGE and VERSION, such as:

<xsl:value-of select="msxsl:node-set($trisoft.object)/ishobject/ishfields/ishfield[@name='VERSION'][@level='version']" />

But none of the custom properties appear to be available in the $trisoft.object referenced above, when using a similar approach:

<xsl:value-of select="msxsl:node-set($trisoft.object)/ishobject/ishfields/ishfield[@name='FDOCUMENTOWNER'][@level='version']" />

Is there another XML structure available to XSL when rendering the Preview window that contains custom metadata?