I have anohter idea which I think is similar to JAP, but without encryption:
Clients which do not have a certain file - act as proxy. That means the client which does not have the file itself, starts to download it, and sends it directly to the client which requested it. This way nobody can say wheather a cleints downloads a file in order to store it on HDD or in order to send it to another client.
This produces traffic which is not necessary, but this has the positive side effect, that you can't say for shure if a client loads a file in order to store it on HDD.