Hi Luke,
Could you perhaps use the Orphan Activities topic area? I would create the forum, leave it visible to students, and move it to the Orphan activities area. This way, students cannot see the forum on the front page of your course, but they can access it from within your lesson.
There's a little bit of information on Orphaned Activities here: http://docs.moodle.org/21/en/Course_settings#Number_of_weeks.2Ftopics
Hope this helps!
All the best,
Lindy