input.xml

<document attribute="some value"/>

schema.sch

<schema xmlns="http://purl.oclc.org/dsdl/schematron" queryBinding="xslt3"
        xmlns:svrl="http://purl.oclc.org/dsdl/svrl"
        xmlns:fun="tag:dmaus@dmaus.name,2022:SchematronFun"
        xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

  <ns prefix="fun" uri="tag:dmaus@dmaus.name,2022:SchematronFun"/>
  <ns prefix="svrl" uri="http://purl.oclc.org/dsdl/svrl"/>

  <xsl:function name="fun:parse" as="element(parsed-attribute)">
    <xsl:param name="attribute" as="attribute()"/>
    <parsed-attribute xmlns=""/>
  </xsl:function>

  <xsl:function name="fun:validate" as="element(svrl:schematron-output)">
    <xsl:param name="document" as="node()"/>

    <xsl:variable name="result" as="map(*)"
                  select="transform(map{'stylesheet-node': document(''), 'source-node': $document })"/>

    <svrl:schematron-output>
      <xsl:sequence select="$result?output"/>
    </svrl:schematron-output>
  </xsl:function>

  <pattern>
    <rule context="document">
      <let name="parsed-attribute" value="fun:parse(@attribute)"/>
      <let name="sub-validation-report" value="fun:validate($parsed-attribute)"/>
      <report test="$sub-validation-report//svrl:failed-assert" properties="sub-validation-report"/>
    </rule>
  </pattern>
  <pattern>
    <rule context="parsed-attribute">
      <assert test="false()"/>
    </rule>
  </pattern>
  <properties>
    <property id="sub-validation-report">
      <xsl:copy-of select="$sub-validation-report"/>
    </property>
  </properties>
</schema>