> the sq_length slot in the weakproxy type is set to proxy_length.

Nice. Its tp_getattro gets in the way of using __len__ directly, but this can be side stepped by manually binding the descriptor:

    class Test(object):
        def __len__(self):
            return 2**31 + 5

    >>> t = Test()
    >>> p = weakref.proxy(t)
    >>> p.__len__()
    2147483653L
    >>> type(p).__len__.__get__(p)()
    -2147483643