NaCl is not needed for that. Embedding CPython into the browser has been done for some time now with Emscripten. Here's a demo: http://syntensity.com/static/python.html
This has the advantage over NaCl that it works in all browsers, not just Chrome. It's also portable.
The point of a NaCL VM would be its speed. It could actually be PyPy or something, for real. Embedded just like the Flash or Java Applet runtime (in the NaCL container, which we have rather better security salts [pun] about.
This has the advantage over NaCl that it works in all browsers, not just Chrome. It's also portable.