So, to clarify, what you want is to manage subscription purely by email?
With the setting Jc changed, it is possible to post a new topic, but I don’t think it is possible to become a full subscriber without logging in once. If I am reading the description “What are staged users” on this post correctly, then the user will not get any other messages in the category (or Announcements) until they log in once and create a full account.
Just to check, @aylward did you receive any email notice for this thread in Support?