I have exactly the same need and use-case as Mathias in my project which includes a requirement to embed python3 in a relocatable folder structure w which serves as an application package (https://github.com/shakfu/py-js).

This can be done using the Framework structure, thanks to Greg Neagle's solution referenced in (https://bugs.python.org/issue42514), but not for any python builds with --enabled-shared.

In any case, providing options (at the c-level or otherwise, for embedded applications as described by Mathias would be ideal: "I can imagine to provide several functions to build up the pythonpath starting from something.
So say, have a 'get python path from argv[0]', a 'get python path from shared python library' and a 'get python path from prefix' function (I may miss a variant)."