Rather than exposing the function pointers directly to the linker, I'd be happier with a function based API, with the pointer storage then being made static inside ceval.c.

/* Each function returns the old func, or NULL on failure */
_PyEval_GIL_func _PyEval_replace_take_GIL(_PyEval_GIL_func take_gil); _PyEval_GIL_func _PyEval_replace_drop_GIL(_PyEval_GIL_func drop_gil); 


The redirection code (sans error checking) would then look like:

old_take_gil = _PyEval_replace_take_GIL(stm_take_gil);
old_drop_gil = _PyEval_replace_drop_GIL(stm_drop_gil);

Currently they'd just replace the statics and would never fail, but it provides looser coupling regardless.