There is a version of this job where you fix it in the grade. We have tried it and we do not like the films it makes.
Waiting costs money and everyone on the floor knows it, which is why the wait has to be scheduled rather than improvised. Our call sheets mark the two or three windows in a day where the sun does the work for us, and everything else is built around them.
The grade is for shaping a film that was already photographed properly. It is not a rescue service.