You can not pick and choose which files you are able to access through the Backup, Archive and Restore window from within the master server. This is why you must be careful as to who you give access to the master server.
Have you thought about using some type of encryption? You will still be able to browse systems and see their filenames, but restoration will not be possible unless the recipient has the encryption keys.
Maybe someone out there who has been using NBU far longer than me has something they do that is a little more granular than just restricting access to the master server.