> How would it work? We would have to add various unions to the
> PyUnicode_Object definition?

No, you'd just need a temporary union defined in unicodeobject.c that would look something like:

typedef union { unsigned long v; char s[SIZEOF_LONG]; } U;

(with better choices of names).  Python/dtoa.c does a similar thing to read / write the pieces of a C double using integers safely.