> I would hope that competent C programmers would know not to read or write to undocumented fields. But if they come from a Python background, that might not be obvious.

Tread carefully when changing decades-old API, documented or not. The docs weren't always as useful and complete as they are now.

> If the advice for something is "use Python", we should probably add an API function.

Not really. If PyObject_GetAttrString doesn't add too much overhead, IMO it's fine to use it.