Can something like this work? Not sure if you could use this but it might be worth taking a look at.
http://www.amazon.com/Iomega-Network.../ref=pd_cp_e_2
Here is what one viewer said about permissions in a review:
"You can set up different permissions for each folder by going to the web interface. The web interface comes up as completely open by default. You'll have to set up an admin password. You can create user accounts for each of the folders and specify under each folder if you want to secure it and which users to allow access to the folder. Creating/deleting folders is a snap as well (though folder names are limited). "
I don't know much about this option or if it will protect what you need to be protected, just figured i would throw it out there in case it proved to be useful.
Best of luck with it.