I would definitely not spray nearly ready flowers with anything!
Since they are close to harvest, I would be cutting off the affected leaves now, this may slow maturation slightly, but letting the powdery mildew spread is far worse.
You certainly can't dry the flowers with the powdery mildew left in place.
Once you remove the affected leaves then harvest later on, and once dried properly (lots of air exchange), curing should be fairly straight forward as the mildew can not survive on dried flowers/buds.
Hope this helps, Organoman.