Right now major parts of the buffer API are broken for non-trivial buffer definitions. IMHO a backport of the fix doesn't count as a new feature although it needs some new internal functions.

I don't quite understand why Nick thinks that ABI compatibility is a challenge. The structs, typedefs and function definitions aren't modified. The new functions aren't visible because they can be implemented as static functions if PyBuffer_ToContiguous() is moved to memoryview.c. That won't break the ABI eiter.

If we want to keep the function in its old place then we can prefix the new functions with _Py and include them in a private header file. That would export new function.