Bookmarklet to quickly switch between a repo and the gh-pages (github.io) view.

gistfile1.js

// Instructions.  Save as a bookmark.  Click when on a repo or github.io (gh-pages branch) site.
javascript:(function(h,p){
  location = /io$/.test(h) ?
    'https://github.com/' + h.split('.')[0] + p:
    'http://'+ p.split('/')[1]+'.github.io'+ '/' + p.split('/').slice(2).join('/')
})(location.host,location.pathname);

Interesting thought. You could circumvent the cross-origin issue by assuming that index.html exists on github.io if it exists on the github.com domain in a gh-pages branch. Have your xhr check that the gh-pages branch exists, search it for index.html, and render the appropriate path prefixed with ____.github.io.