Taskdef and custom classloader

With a Groovy based build tool I have the following problem.

The JVM this build tool is running in, is started with a jar in the classpath that contains a certain Ant task (groovyc). What I need now is a way to define a groovyc task from _another_ jar with a different version.

I try to do this the following way:

<taskdef name="isolatedLoader" classname="org.gradle.api.tasks.util.IsolatedLoader"/>
<isolatedLoader ref="loader">
      <classpath path='path+with_other_groovyc_and_ant'/>
</isolatedLoader>
<taskdef name="groovyc" classname="org.codehaus.groovy.ant.Groovyc" loaderref="loader"/>

The lsolatedLoader class looks like this:

class IsolatedLoader extends MatchingTask {
    private String name;
    private Path taskClasspath;

    /**
     * sets the name of the reference which should store the Loader
     */
    public void setRef(String n){
        name = n;
    }

    public void execute() throws BuildException {
        Project project = getProject();
        AntClassLoader loader = new AntClassLoader();
        loader.setClassPath(taskClasspath);
        loader.setParent(getClass().classLoader.systemClassLoader.parent))
        project.addReference(name,loader);
    }
...
}

When I now execute the groovyc task, Ant can't find it.

If I try a different way of achieving my goal, this is when I define the IsolatedLoader like:

public void execute() throws BuildException {
Project project = getProject();
AntClassLoader loader = new AntClassLoader();
loader.setClassPath(taskClasspath);
loader.setParentFirst(false);
project.addReference(name,loader);

I do get ClassCastExceptions.

Has anybody an idea how I completely isolate a taskdef from an existing classpath?

- Hans