So, the obvious thing felt like something like...:
<xsl:template match="namespace-uri() = 'http://www.w3.org/TR/xhtml1/strict'">
<xsl:copy-of select="."/>
</xsl:template>

...and I've tried numerous variations over this theme over the past couple of weeks, but it doesn't "work".

You can only match on match pattern, not arbitrary XPath
expressions. Try
 <xsl:template match=*["namespace-uri() =
  'http://www.w3.org/TR/xhtml1/strict']">