Andrew Ng's ML-Class (Stanford, 2011) was not an example of a true MOOC
- it was massive at 70k students
- it was not open in that the subject matter was prescribed
- it was online
- it was a course
But it serves to make the point about levels of engagement... the student could engage at a Basic level (watch the videos, do the review questions) or at the Advanced level (watch the videos, do the review questions, do the assignments); and they could switch between Basic and Advanced at any time.
The way this desirable feature could be incorporated into conventional e-learning programmes would be to allow guest students to "sit in". If they enjoyed the experience, then next time around they could enrol proper.
It's like you're mountaineering: if you aspire to one day gaining the summit you might like first to be one of the team that carries the gear to base camp. You get to meet the gun climbers, you get to look at the mountain close up, and you can start to gauge your own fitness and preparedness.