A friend of mine has a draft cross mare who is IR and the vet (and nutritionist) that looked at her recommend putting her on dry lot, soaking her hay for a minimum of two hours before feeding and put her on a supplement regime. It seemed to help a lot, even though the mare was getting less work on this diet she actually dropped weight and her attitude got better. The vet thought that after a couple months they might not have to soak the hay and they basically had to reset her metabolism because she was sooo overweight.

good luck, having a special needs horse can be complicated...