In your PhD courses, though, everyone knows enough to be an expert of sorts. There's no one posting "How do you do Question 1?" followed immediately by that same person posting "How do you do Question 2?", right? In my experience with undergrad classes, about 85% of the posting volume is that sort of thing. (I had a bunch of classes with a message board component.)
You also need multiple experts, because the conversation between experts is the interesting part. And because nobody ever knows everything, so the disagreement between experts is really the interesting part. With only one expert, particularly an expert with grading power, the expert tends to become God, which isn't good for the expert or the group.
If you picked a topic that the participants had some personal interest in (as opposed to "I need to fill gen ed requirements" or "I need this for my major" or "I needed another elective and this one looked easy"), didn't artificially limit the participant pool, and didn't grade, I think you'd end up with a good online community - but I don't think you could call it "college." I don't doubt that the participants would learn a great deal, though.
