I was thinking of a subclass of an existing class, both implemented in C.  collections.Counter does that, but it uses _PyType_LookupId() which is in the limited API.

Would it be possible to achieve equivalent functionality by using _PyType_LookupId()?  If so, maybe instead of the proposed new function PyType_GetSlot, we should make _PyType_Lookup and _PyType_LookupId part of the public API.