How to implement private categories & public categories

Question

I use q2a, integrated with my wordpress site (ascendly daught com).  The Q2A area is primarily intended for parents and teachers, which will be registered users in wordpress.  I've currently limited the QA site to only registered users, which is good enough,  but I would really like to have a section that was totally open to the public.  What would be the smart way to approach this?

Has you seen this done by somebody else?

I'm guessing I could write a plugin that lets me flag a category as public vs. private (ie, viewable by 'anybody' vs. viewable by 'registered users' - does this sound possible?

I'm think that I can _maybe_ implement this via a theme, but I'm concerned that this approach isn't secure enough.

Basically, I'm fishing for direction  - how can I have a public section along side a private section

Tell me what you think - any thoughts are appreciated

Comments

Another option might be to have two separate installations, maybe with some shared tables?  Not sure if that is feasible.

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I am interested in something like this. Rather what interest me is that there is group that user belongs to and question to be visible, allow answers votes etc to a selected group of users.