Enabling Ashcroft from within Ant is easy. You probably already have a target running a <junit> task somewhere in your build script. (If not you're in deep trouble). Ok, just amend it a little bit like this:

<junit fork="yes">
  <jvmarg value="-Djava.security.manager=com.thoughtworks.ashcroft.runtime.JohnAshcroft"/>
  <jvmarg value="-Dcom.thoughtworks.ashcroft.runtime.forgiving=true"/>
  <jvmarg value="-Dcom.thoughtworks.ashcroft.runtime.logging=report.txt"/>
  <classpath>
    <!-- In this classpath you must include the ashcroft jar file -->
  </classpath>
</junit>

It is imperative that you run JUnit in a forked JVM, otherwise the jvmarg will be ignored, and Ashcroft will not be activated. For maximum speed, we strongly recommend you also specify forkmode="once" (requires Ant 1.6.2 or higher).

Also see Ashcroft Configuration for a full list of configuration options.