We've actually created one for online editing but it is undocumented. We'll keep this issue open for the day we will add this to the 2.0 API. You can discover how it works by watching the requests to the Online Editor. However, we will certainly change the behavior of this API and we wouldn't recommend using it at this time except for experimental purposes.
Open Science Framework has same problem as Apiary: OSF supports GH repos via their read/write API and would like to support BB repos via similar mechanism (i.e., just refactor the GH code to reflect API differences).