But please get out of the habit of using xsl:value-of
when you mean xsl:sequence, and please get into the habit of declaring the
types of your functions! This should be

The cheapest solution is probably a text or comment node rather than an
element, something like:

<xsl:function name="my:getNode"><xsl:comment/></xsl:function>

  <xsl:function name="my:getId">
    <xsl:value-of select="generate-id(my:getNode())" />
  </xsl:function>

Remember that an LRE like <node/> might be creating a lot of namespaces...