build.xml

<project name="Test" basedir="." default="build">
  <taskdef name="schematron" classname="name.dmaus.schxslt.ant.Task" classpath="target/ant-schxslt-1.9.5.jar"/>
  <target name="build">
    <schematron schema="schema.sch" file="document.xml" report="report.xml"/>
  </target>
</project>

document.xml

<document/>

schema.sch

<sch:schema xmlns:sch="http://purl.oclc.org/dsdl/schematron" queryBinding="xslt3">
  <sch:pattern>
    <sch:rule context="*">
      <sch:assert test="false()"/>
    </sch:rule>
  </sch:pattern>
</sch:schema>

Ah! Fixed that last one. Martin had warned me you sometimes need to specify fork="true" when calling Saxon via in ant. Sigh.