As you can tell, I'm no cryptographer and if nested hashing makes sense, I'm happy to change that.

As for a lower work-factor bcrypt/pbkdf2, we'd have to lower that so dramatically that we'd lose much of their value.

As per @timgraham's suggestion, I've moved the discussion to the django-developers mailinglist: https://groups.google.com/forum/#!topic/django-developers/KKXDyeJLowk