I think the closest thing to a solution to your problem that *I* have heard about is activity locking. Activity locking is planned for Moodle 2.0, but in the meantime there are some folks who have done some work in this area and contributed code. You might search for "activity locking" or "conditional activities" and see what you come up with (both in forums and in the modules and plugins database)
There is a forum on conditional activities. If you haven't seen it yet, you might take a look!
atw
Courses and course formats
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