It's not possible in the GUI- the feature is disabled when the vApp is started,
but with the vCenter Orchestrator plug-in for vCD it should be possible (http://www.vcoteam.info/learn-vco/vcloud-director-vapp-hot-clone.html)
That's why I suppose that it should be possible with the API.
Thanks for the hint, I will post the question also here: http://communities.vmware.com/community/vmtn/developer/forums/vcloudsdkjava.