I haven't started really looking into it yet, but I found this blog that looks like a good description of what exactly happens during stack unwinding in python and gets a large part of the way to resuming execution in pure python without even any native code: https://web.archive.org/web/20250322090310/https://code.lard...
Though the author says they wrote it as a joke and probably it is not possible to do robustly in pure python, but I assume it can be done robustly as a patch to CPython or possibly even as just a native C extension that gets loaded without people needing a patched build of CPython. If you know any good resources or information about how to approach this, or start working on it yourself, let me know.