Changing the subject because I think it's worth talking about over arching goals and principles without too much implementation detail. (Although obviously goals must be achievable too.)
I suggest that the backwards compatibility WHATEVER has to be shipped with standard Python, ie the link you click on on the main www.python.org website.
And any code or build setting changes are made by the extension author should likewise be made at the source, and not require any action by distributors or users.
The goal of backwards compatibility is not to break things, to keep existing code working for as long as practical. So the necessary work should be done by as few as people as possible. Someone building a standard OS image for their workplace, or maintaining the local wheel repository for the organisation, should not have to do anything. Even Linux distro packagers won't thank us for adding one more step that has to be done.
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cheers,
Hugh Fisher