maven-script/maven-script-ant/src/test/resources/test.build.xml

<project>
  <target name="init-cp" unless="cp-exists">
  	<path id="maven.plugin.classpath" location="path-is-missing"/>
  </target>
  	
  <target name="test">
  	<condition property="cp-exists">
  		<isreference refid="maven.plugin.classpath"/>
  	</condition>
    <antcall target="init-cp" inheritall="true"/>
  	
  	<property name="cp" refId="maven.plugin.classpath"/>
    <echo>plugin classpath is: ${cp}</echo>
  </target>
</project>