Can the rpython translator support js as a target? Could you compile pypy to js that way, to provide a python environment in existing browsers? To be honest though, JS is a perfectly OK language for web browsers, and if you really want to reuse python code it probably makes more sense to translate it before it gets to the browser, using automated tools or otherwise. Supporting everybody's favourite language natively…
Note that there is also a working implementation of Javascript that runs on PyPy. This project was actually an implementation of Python that compiled to Javascript as a target.