Hmmm, this is an interesting one.
You could house the discussion inside a course in moodle, and use an enrollment key for the course. Students who request access will have to be enrolled (I know, a few more minutes at the keyboard mindlessly clicking for some poor Moodle admin like myself). You can still have a link to the course from the web page, but "students" that are not enrolled will be asked for an enrollment key, even if they can log in to your moodle site, so they won't be able to get to the forum. Students who are enrolled will be asked to log in to moodle, but then taken straight to the course page.
Teaching with Moodle
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