Kitten_val--- if you find a system, please share. I'm in the same situation. I went to watch a schooling show just to see what they wore and how it went.
The popular combo was a heavy, but fitted black or dark coat, turtle neck, Iridon winter breeches with a layer of silks under all that. The whole place was a walking catelog for Mountain horse boots. =) Ear muffs seemed to be accepted.
Same setup, warmup outside (on the top of a hill of all windy places), test in the blasted cold indoor.
Here's to icicle dressage tests.
If your horse says no, you either asked the wrong question or asked the question wrong
And God took a handful of southerly wind, blew His breath over it and created the horse