The mare in my Avatar is a non-sweater. We have tried One AC with no success. However, it has worked for some other people on the board. We even tried dark ale beer, as we were told that can help. Gave her a GREAT shine to her coat, but no sweat. We haven't come across anything yet that has allowed her to sweat. We still show her. We are just careful with her in the hot weather and keep her under fans, etc. She has an opposite work schedule of everyone else. She is worked and made fit in the late fall/winter/spring. Then I am able to coast her through show season without working her in the summer heat, as she comes out of winter fit. My "normal" horses are like everyone elses and fitted up during the spring/summer.
We just take extra careful measures with her. Fans, several hose downs a day in hot weather, etc. I feel very bad for her because I am sure it's not pleasant to not be able to sweat. But, we make her as comfy as we possibly can. And anyone who knows the mare, knows she gets everything she needs/wants ;o)
~Jen~