So what *will* result in a namespace declaration being output?

Let the XSLT engine worry about outputing the namespace declarations - just
do

<xsl:transform xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
               xmlns:xsi="http://www.w3.org/2000/10/XMLSchema-instance"
               version="1.0">
   <xsl:template match="foo">
     <xsl:element name="bar">
     <xsl:attribute name="xsi:noNamespaceSchemaLocation">
       [somewhere]
     </xsl:attribute>
     </xsl:element>
     <xsl:apply-templates/>
   </xsl:template>
</xsl:transform>

Hey, it works! Thanks! I knew I was making it more complicated than
it had to be.