Leovet have just brought out a coat spray with UV protectors. You can buy one for dark horses and one for greys. Don't know if it's any good as have not tried it myself.
I suppose the best way is to keep the horse covered in a fly/UV rug but it's not easy finding one that doesn't rub shoulders, rub mane out, etc. If you weren't showing I don't suppose it would matter that much. Thermatex do an Atlanta fly sheet which is a really light mesh and looks stretchy, and I know a few people that show who use them as they don't rub.
Have to agree, the bay in your sig pic looks a real headturner. Who is he/she?