What's the maximum level of privacy a user can get within the site?

Continuing the discussion from Is it possible to hide the user listing page:

I’ve got a bunch of users who may want to remain within a private subset of the entire user population.

They will have exclusive access to a private Category.

If I:

  • disable the User Directory
  • disable user cards for anonymous users
  • keep the group private

… I presume all members will still be able to view these users details if they happen to work out one of their usernames? (Let’s assume none of them post in the ‘public’ area of the forum)

What additional steps, if any, can I take this to lock this down to the max so these users are kept as private as possible and isolate them from the general user population?

(and yes I have thought about keeping a private site just for this set of users, but the downside is I want to leverage the facilities of the forum for a wider audience … and what a waste of a rather tasty domain name that would be! :)).