creating an access level between public and private?

I recently began working on an archive that uses Omeka. The archive contains some culturally sensitive information. Historically, the sensitive material has been protected by being kept private. Because we have controlled the creation of new accounts on the back end of Omeka, this has worked for us.

However, as our archive grows we would like to start using guest/user/contribute plugins. We also have a lot of new collections that we would like to hide behind a registration wall by making private and requiring guest user registration, similar to this strategy: However, we do not want the sensitive collections to be visible to everyone who creates an account! We really need an intermediary designation between the public/private binary--call it protected--that would be tied to a certain user roles OR the ability to password protect certain collections.

For example--"researchers" could view public and protected collections, but would not be able to view private collections. Only superusers could view private collections.

Or: All private collections would be visible to registered users but some collections would require a password to access. Registered users would have to contact our archive to request the password.

If faced with this issue from the beginning, I would just likely not digitize these items and instead require that individuals come into our location to view the items, but the problem is that the sensitive collections have already been digitized and uploaded and used in this manner. I don't want to delete them and undo the work, but we also need to implement the guest/user/contribute plugins.

Is there a plugin (or combination of plugins) that would create what I need? Any suggestions are much appreciated!

Note: I am working on an archive whose stylesheet is controlled by our parent institution--because of this, I do not have direct access to the code. We would prefer a plugin solution over modification of our stylesheet, since we share our Omeka theme with other departments. We can get someone to write a plugin, but I need to know if Omeka has the capacity to do this first.