In CPython 3.7 it was possible to do:

#include "pystate.h"
...
PyThreadState *ts = PyThreadState_Get();
PyInterpreterState *interp = ts->interp;
interp->eval_frame = my_frame_eval_func;

This is no longer possible because in 3.8 the PyInterpreterState is opaque, so, Py_BUILD_CORE_MODULE needs to be defined defined and "internal/pycore_pystate.h" must be included to set PyInterpreterState.eval_frame.

This works but isn't ideal -- maybe there could be a function to set PyInterpreterState.eval_frame?