In our specific case the value would be an offset added to the PyObject*, and there we would find a pointer to a C function (together with a 64-bit signature), and calling that C function (after checking the 64 bit signature) is our final objective.
And what the C function does really is faster than the lookup through a dictionary? I find that hard to believe.
I still think this is out of scope for python-dev. If this is something you want to be able to do for Python 2.4 as well, then you don't need any change to Python - you can do whatever you come up with for all Python versions, no need to (or point in) changing Python 3.4 (say).
We can go ahead and hijack tp_flags bit 22 to make things work in existing versions. But what if Python 3.8 then starts using that bit for something else?
Use flag bit 23 in Python 3.8. You know at compile time what Python version you have.
As this is apparently only relevant to speed fanatics, too, I suggest that you check how fast PyPI works for you.
Did you mean PyPy?
Oops, yes - Freudian slip :-) Regards, Martin