Do you mean  "if (!Py_UNICODE_ISLOWER(*s)) {"  (with the '!')?

This sounds fine to me, but with this approach all the uncased characters will go through a Py_UNICODE_TO* macro, whereas with the current code only the cased ones are converted.  I'm not sure this matters too much though.

OTOH if the non-lowercase cased chars are always either upper or titlecased, checking for both should be equivalent.