I really don't know much about getting weight off of a horse, since I have a typical TB (just trying to keep weight on him!), but I have heard that if you turn a horse out for anytime at all without a muzzle they will make up for what they didn't eat earlier. For example I knew someone who decided that to slim down their horse they would only turn it out at night. The only thing they got was a fat, grumpy horse (because there was a lot of activity around during the day and he ate all night - the horse never slept - very, very grumpy).

But with that said, if they don't move around then they won't be burning many calories. Horses acctually walk quite a lot while grazing.
My best friend put a grazing muzzle on her FAT Arab one summer (because my friend was pregnant and couldn't ride and work the fat off of the horse) and was very pleased with the result. **Remember that you will need a break away or leather halter to attach the muzzle to. Sorry I couldn't be more help! Good luck.