まだ試してないけど、それっぽいコードをみつけた。
後で忘れないためにメモメモ。
FileObject scriptToExecute = projectDir.getFileObject("build", "xml");
DataObject dataObj = DataObject.find(scriptToExecute);
AntProjectCookie antCookie = (AntProjectCookie) dataObj.getCookie(AntProjectCookie.class);
AntTargetExecutor.Env env = new AntTargetExecutor.Env();
AntTargetExecutor executor = AntTargetExecutor.createTargetExecutor(env);
Properties props = new Properties();
env.setProperties(props);
ExecutorTask task = executor.execute(antCookie, new String[]{"clean"});
task.result();
後は「org.apache.tools.ant.module.api.support.ActionUtils」かなぁ。