When it's warm out, I just open the doors of my barn and the wind is usually enough to have my mare dry in a matter of minutes.
However, it's cold now and that would turn her into an ice cube. Do you own a leafblower that you could desensitize your horse to? My friend did that when she decided to give her filly a full bath in December and apparently didin't realize how thick her winter coat was, so she dried her with a leafblower.