Is there a way, perhaps witha plugin, to restrict posters to a public topic to a defined list? Ideally this should be a topic specific list of users.
Not sure exactly what you are asking for, but category
security settings is the place to look to set up who is allowed to create topics, reply to topics or even see topics within each category. So I guess you’d want to put the topic you want to restrict access to within a category set up with the permissions you desire. E.g.
everyone can see and reply, but only
staff can create.
That is at a category level. What about at a topic level?
Convert the topic to a PM if topic-level restrictions are needed. Topics do not support restricted access except that provided at the category level.
Or just create a category for the one topic if you need it to be public but restrict who is allowed to post to it. You could then later close the topic and move it to a different category and delete the special category with restricted access.