Many, if not all, show people vacuum their horses. It's a much better alternative to bathing.
The first thing you would desensitize your horse to is rubbing him all over with the hose off. Getting him used to the long black tube being around his legs and under him.
Then you'd desensitize him to turning on/off the machine. Over and over and over.
Then you'd desensitize him to sucking the air close to his shoulder, then removing the hose. Get closer, if he remains calm, then take the hose away. Closer and closer until you put it on his skin.
I imagine how long the process is dependent upon how good your horse is to being desensitized.