My idea was a little bit different: I would drop in a single file to my Jekyll repo that adds a "Create a new post" page on my blog. There is a text field with a live markdown preview and a "Commit" button that would make the calls (via javascript) to the Github API to create a new commit in the repository. This commit triggers the hooks that rebuild the blog.
The main hurdle was authentication, since Github OAuth doesn't support any client-side flows (which is understandable since it's not a completed spec, to my knowledge). So I would have to pass my user/pass in everytime.
But I got the API calls all working (after Github support helped me get CORS working), but I lost interest in the project and it's just sitting around collecting dust
The code: https://github.com/swanson/lanyon/tree/gh-pages
Demo that doesn't quite work: http://swanson.github.com/lanyon/