BarryO I don't know. Southeastern Michigan Dahlia Society, had a ballot box at their show for the public to decide. Their 2025 flower of the year is Ketchup & Mustard. It will be easy for the society to offer enough of this cultivar to everyone who would like it.
For the larger shows, it would be nice if they could announce the President's choice, Flower of the year at least one year before the show.
Challenge classes are usually sponsored by a society or an individual, so that is a little harder to request in advance. I think one can get a good idea of what the challenge classes might be based on the year prior show booklet. For example, Michigan Dahlia Society always sponsors the Midwest origination challenge section for the Midwest show.