Is there a way to allow Anonymous/Not-Logged-In-Members to post topics and/or replies in designated categories only? Category security allows create and reply for everyone. This ability allow me to host events and have attendees respond to what’s happening in real-time without gaining access to the rest of the forum. They are now exposed to the forum and may become members.
No. That would allow anyone to post to your forum without even validating an email address, which most people consider an unacceptable spam risk.
So make the rest of the forum requite being members of some group (easy if you are managing subscriptions somehow) and then have the events category allow everyone. This will let them in the forum, but not have access to the other categories.