#define _Py_ONCE_VAR(var_decl, var) \
      var_decl var = NULL;
      static _Py_OnceVar var ## _once_meta = {.next = NULL, .obj_ref = (PyObject **) &var}

Yeah, I had a similar idea, but I fear that it will increase the usage of the C stack memory.

See the issue #28858: my old _PyObject_CallArg1() macro allocated an implicit array on the stack and increased the stack usage, whereas the purpose of the macro was to *reduce* the stack usage (just the opposite!). So I removed the macro.