Import tagging is done in the Configure Auto-Import... area, on a per-folder basis. You can have MC apply values to any number of tags upon import (and optionally to omit applying a tag when the tag already exists). The tagging mechanism uses the MC expression language so you can be creative. So, you can use directory components to help assign tags (e.g. Artist, Album), or apply static tags as desired, based upon, for example, Media Type. Ask for more specific help where you need it. In your case, since all your Unreleased files are in a given directory tree, there's probably no need to add any sort of Unreleased tag or keyword. The file location alone might be sufficient.
I'd use a panes view that shows all Audio files from any location, and it would have a Stage column showing values of Released and Unreleased. A second pane might be a location-based pane, that shows the full or relative path of the media, perhaps in a hierarchical tree, so that you can explore it just like you do in Explorer mode. The files list gives you the added benefit of being able to see files for any number of sub-folders, so you can tag entire lots of media for a given Artist, for example.
I'd customize the file list view, and save it as a Preset, and have it show all the fields you require to be filled out. That way, you can see what's missing at a glance, or by quick column sorting. Perhaps also include a column that makes clear which files are Unreleased.
As far as not seeing the view from Clients - the only thing I can think of is to use Access Control on each client, but I don't know if you're able to enforce that. It would be nice to be able to protect content on the server, and by default require clients to enter the access control password. Regardless, the view would be seen, but the content would not, so in the case of a pane showing Released and Unreleased, it would only show Released. I'd make it a more clever expression pane, showing the path location for Released media, and a prefixed path location for Unreleased media (e.g. *UNRELEASED*\<media type>\<artist>\<album>). This way, you can have a useful view regardless of stage of release.