I too have been looking for a solution to this problem for quite sometime. I would like to see a system where users on my blog could create directories and upload files to them which would be seen by the administrator and/or any group of users I define (as the administrator).
My problem is that in addition, I would need for every file transfer to be secure AND for the user to have already been authenticated as a function of having signed into the wordpress backend already (to avoid a second login screen which is just annoying).
In this way, users could share content in a more robust way that is not necessarily visible to casual browsers of the site itself.
Please, If anyone has thought, it would be much appreciated. Thanks in advance!