I am in a very similar boat. The horse I lease is an easy keeper. He has been overweight since I ever met him, six years ago. I got him to shed a fair amount of weight by riding him a LOT. He never has showed signs of laminitus, but has the tell tale cresty neck and butt pillows of fat of an insulin resistant horse, so care must be taken.
He is on a dry paddock now for 5 months. He has lost a lot of weight, and looks like a normal horse now. (I am so proud of him!). But, even with a slow feeder, he ends up with no hay for many hours at a time, every day. He is always hungry and thinking about food.
I take him out almost every day for about 40 minutes on the grass. This has helped tremendously with his sanity. Just being FREE to move, to 'seek' the good grass, to nuzzle and pull the tastier bits, ignore the not so good ones. He LOVES it, and after this, he is so much happier to go for a ride.
I also reason that it gives him nutrients , like micro vitamin and minerals, that he cannot get from dried hay of a single species. If we are able to put him back in the herd, to open graze, he will need to maintain a familiarity with grass.
What I mean is that if you take a horse that almost never gets grass, and put him out on the grass, he will get sick, quite likely. If he gets SOME grass on a regular basis, he will be more able to make the switch . . . . or so goes my rationale. I could be wrong, but that horse is SO happy to see me when he knows he will get his free time on the grass. I simply MUST indulge him his little pleasures.