Whoo! I think I managed to work it out on my own. The key and template
on the stylesheet change to...

<xsl:template match="svg:text[.=('data')]">
(Look for parent elements with our data placeholder)
	<text>
		<xsl:copy-of select="@*"/>
		<xsl:value-of select="key(.,../@id,doc('textsrc.xml'))"/>

(And then this key (matching to the @id in the parent of our svg:text
element) gives us back the contents of the text file parent element
... which is the values of the children elements)

	</text>
</xsl:template>