Dave
I think you're encountering the same issue that a few of us have encountered, and tried to address in the forums and bug tracker. See the discussion here. In my case, I have a site-wide role called AllStudents that contains every student in the school. We have guest access open on nearly all courses, because we want people to see what's going on in others' courses. I use the AllStudents role in various courses where we would want all students in our school to be able to participate (in a forum, in a feedback activity (survey), etc.) If you check out the details in the bug I logged [MDL-37743], I was able to get 99% of the way there by properly permissioning the AllStudents role with the necessary permission to post to the common forum - WITHOUT enrolling them all in the course. The problem I ran into was that the students could do everything they needed within the forum, except reply to a post. The Moodle code explicitly prohibits replies (not posts, edits, or even deletes) unless the user is enrolled in the course.
If, after reading the details, you agree that there are reasonable cases for when users who are not enrolled in a course, once properly permissioned, should be able to participate in a forum, please vote for the bug.
Thanks
Joe