Example of C code that I added to _testcapi:
---
static PyObject *
get_caller_locals(PyObject *self, PyObject *Py_UNUSED(args))
{
    PyFrameObject *frame = PyThreadState_GetFrame(PyThreadState_Get());
    if (frame == NULL) {
        Py_RETURN_NONE;
    }
    return PyObject_GetAttrString(frame, "f_locals");
}
---

Python example:
---
import _testcapi

def f():
    x = 1
    y = 2
    print(_testcapi.get_caller_locals())

f()
---

Output on Python 3.11:
---
{'x': 1, 'y': 2}
---

=> it just works.

A PyFrameObject is a regular Python object, you can use functions like PyObject_GetAttrString().

Maybe I missed something, correct me if I'm wrong.