I was working on something similar to this (sorry, no showcase). There's a jQuery extension included for working with contentEditable text selections and formatting text with tags. I think it does a better job with overlapping tags and it merges anchor tags with same hrefs correctly. It does not work on IE < 9 (I think IE9 supports the gecko contentEditable api). You might find the code useful .
Isn't the point of this approach to be _difficult_ to switch back and forth? Next thing you know there will be a chrome extension that swaps your hosts file and you will be separated from procrastination by a single click.
It's silly how this is actually becoming the truth. People use the same password and different usernames for different sites. In the end, remembering your username becomes the problem :)