I guess one workaround could be this: instead of doing your file deleting in a POST script, add another node that does this. If the node job is submitted similarly to the process jobs, it should end up with and NFS id of nobody:nogroup, and thereby be able to delete the files. In this model, you'd have a delete node that would immediately follow each process node. If you do that, the delete node will only be run if the process node succeeds.
You might find category throttles useful for your workflows:
http://research.cs.wisc.edu/htcondor/manual/v8.2/2_10DAGMan_Applications.html#SECTION003108400000000000000