Here's a new version that implements __build_class__ in C.  It's still an ordinary built-in function loaded using LOAD_GLOBAL so you can play games with this; however I'd rather keep that an implementation detail and not a feature (unlike overriding __import__, which *is* a feature).

One thing that's missing is support for __metaclass__, either in the module, or in the class.  I don't want to support those (maybe the PEP should mention this), but we need a fixer for 2to3 to warn about this, at least (and to convert class C: __metaclass__ = M; ... into class C(metaclass=M): ...).

BTW, the test_grp failure mentioned earlier is bogus; that's an artefact of Google's LDAP.  Some more tests probably fail, but I'll worry about those later.
File Added: meta.patch