It would be trivial for a user to layer some encryption on top of the service, preventing anyone on the network from doing any analysis of what they are transmitting.
wouldn't the hash be different for every rar password used?
I guess the problem is once any copy infringing hash is found it is trivial to search the network and find everyone who has transferred it?
edit: although if the only writer is the uploader, would you be able to tell who read the copy infringing copy vs who just has it because their part of the network?