After sleeping on it I was able to answer my own question, however I suspect
that that p1 and p2 templates could be generalised to handle any named
parameter, but haven't figured that out yet.

<xsl:template match='p' mode='parameters'>
    <xsl:param name='caption'/>
    <xsl:variable name="@name"/>
    <xsl:value-of select='$caption/p[@name=$name]/@value'/>
</xsl:template>

<caption key='Client not on file'>
 <p name="client-no" value='123456'/>
 <p name="priority" value='99'/>
</caption>

Cheers,
Wendell

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Wendell Piez                            mailto:wapiez@xxxxxxxxxxxxxxxx
Mulberry Technologies, Inc.                http://www.mulberrytech.com
17 West Jefferson Street                    Direct Phone: 301/315-9635
Suite 207                                          Phone: 301/315-9631
Rockville, MD  20850                                 Fax: 301/315-8285
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  Mulberry Technologies: A Consultancy Specializing in SGML and XML
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