Has anyone tried? How long would it take? How marketable would such education be? Could we argue that if you are the a self-starter kind of person you may get a “better” education this way?
Of course no diploma would be given at the end, and perhaps there are nominal fees.
If you want to work for yourself, this could be a good option. If you gain a good reputation working on open source projects, then maybe you can get into smaller companies or startups that aren't as stringent in their policies.
The first is that most people can learn much more and more quickly when interacting with others and with good mentors. Depending on your personality, you might need this less or be able to get some of that from communities online (e.g. forums and open source projects).
The second part is basically the value of signaling and credentials. Part of the value of a Harvard/MIT degree is that it tells potential employers that you were able to get into Harvard or MIT. Since they're very selective that is a useful piece of information for them. A capable seeking person who comes across well in interviews but is self-taught might rationally lose out to an equally smart seeking person who also has a fancy degree.
You could consider Georgia Tech's OMSCS program. It's not free but it's pretty cheap and you get a degree from a well respected school. FWIW I'm an alum of that program.
Online certifications seem to be a negative though. Doing a MOOC in itself signals you have the passion, whereas getting a certification implies you neither qualify to actually go to MIT or Harvard, nor do you have the passion to finish without being motivated by e-paper. While they are hard to get, I doubt the pain level it takes to finish is anywhere near that of a MIT degree.