> Would it be possible to use a "container" object like a Py_buffer? Is there a way to customize the code executed when a Py_buffer is "released"?

It looks nice idea!  Py_buffer.obj is decref-ed when releasing the buffer.
https://docs.python.org/3/c-api/buffer.html#c.PyBuffer_Release


int PyUnicode_GetUTF8Buffer(PyObject *unicode, Py_buffer *view)
{
    if (!PyUnicode_Check(unicode)) {
        PyErr_BadArgument();
        return NULL;
    }
    if (PyUnicode_READY(unicode) == -1) {
        return NULL;
    }

    if (PyUnicode_UTF8(unicode) != NULL) {
        return PyBuffer_FillInfo(view, unicode,
                                 PyUnicode_UTF8(unicode),
                                 PyUnicode_UTF8_LENGTH(unicode),
                                 1, PyBUF_CONTIG_RO);
    }
    PyObject *bytes = _PyUnicode_AsUTF8String(unicode, NULL);
    if (bytes == NULL) {
        return NULL;
    }
    return PyBytesType.tp_as_buffer(bytes, view, PyBUF_CONTIG_RO);
}