At the risk of giving away a million dollar idea; nobody's gonna wanna really edit things in the browser. Too risky and missing features of whatever their favorite editor is. But how about letting people push their code up via git? So you could have a runnable browser window open, push your changes up to your runnable repo and then hit run? If this does turn out to be the key that breaks this open please remember it…
Heroku started out as "develop Rails in your browser", but quickly pivoted into "host Rails in the cloud".