I understand users' need for this, but as a this is your code, why don't you also push it to the gh-pages branch? That would make the exact same thing available at http://reidrac.github.com/canvas-raycasting (and then add a link to your README)
I'm not saying it's useful to host your own demo (although it could be a temporarily demo deployed on demand). I think 5minfork is cool if you want to try a project and the repo owner didn't put a demo website, that's the problem the author wants to solve and I think it's a great idea specially if you can't get the code and try it locally (mobile browsing?).