For this use case, it would be most convenient if there was a C API for creating a sentinel. So in Python, you’d do builtins.sentinel("my_sentinel"); in C you might do PySentinel_New("my_sentinel") or something of that sort.
functools.Placeholder (which Adam mentioned above) is also defined in C and might use this. Maybe third-party C extension libraries like numpy would have additional use cases, but I haven’t looked.