More granular permissions for comment posting

I would like to be able to assign authenticated users the right to create comments against threads in a forum where only a small group of users can create threads while also assigning rights to those same users to both create threads and post comments in other forums.

According to the "Permissions information" included in the "edit forum" page, users need:

• create forum topics permissions AND Post to be able to create forum content, and
• the post comments and (probably) post comments without approval permission AND Post to be able to post comments/replies;

My expectation, before reading the above information, was that any users with View and having the post comments permission would be able to post comments on the threads there. Most of the forums at the site I've put together are for open use by all authenticated members, thus they are allowed to both create topics and comment on any of them but there is also one forum reserved for announcements by the core team, where only users with the "CoreTeam" role can create threads but any authenticated member should be able to discuss these announcements by posting comments against them.

Unfortunately, because authenticated users require the create forum topics permission in order to be able to create threads in the non-announcement forums, if we give them Post in the announcement forums (so they can create comments) they will automatically be allowed to create threads there too, completely defeating the intended "announcement only" nature of the forum.

Thanks for your time,
:-j(enni)