A hint for putting your coat on at the last moment when you’re hot and sweaty from warmup: baby powder!
I rode at Poplar (Atlanta-ish) this weekend, and was already dressed in my sleeveless shirt and on my horse when they waived coats. When the time came for my coat I hopped off briefly, slathered on the powder on both arms and hands, and slid quite handily into coat and gloves. Had to do a good deal of brushing off before I got back on though 
Had I been thinking I would have worn the short sleeved shirt, because they were predicting high humidity and record breaking temps. I would definitely have ridden without a coat if I could have - the true professionals certainly did!
They were just about begging people to leave off coats at Poplar too. Heat stroke is no fun, BTDT, and I know I don’t ride well when I get overheated, which I do easily. I’ll take being fair to my horse (and myself) over fashion any day. I do love the look of coat and stock tie, and I will wear it religiously up to a point. I’d hate to see the dress requirements change too much, but waiving coats is just smartshow management.