Petri Lehtinen runs a clone at https://github.com/akheron/cpython which uses more advanced tricks like hg-git and his own git-hg wrapper to allow him to do this, AFAIK. If this path is important for you, contact Petri and he can provide guidance on how to set it up.

For python/cpython I really wanted the simplest flow to reach a read-only stage, because I lack the necessary git/hg-fu to set up and maintain something more complex "semi-officially". I think the most common workflow is to do small changes vs. Mercurial anyway. When I'm working on longer-term patches I do them in my Git mirror (because I prefer Git branching) and then apply & test patches onto a Mercurial R/W clone before committing.

Thus, for most users I think the read-only mirror is preferable because it's much easier to use and reason about. All writes go solely through Mercurial, so it's clear where the official source is :)

Eli