I just looked at a picture of the E-Z feeder, and although I have never used one I HAVE used feeders to slow the horse's intake of hay. You say that you want to know if it would be helpful for soaking hay. Do you want to soak AND slow the eating, or just soak the hay? If you only want to soak the hay I suspect that the E-Z feeder would make it harder b/c you'd have to pull the soaked hay out of the water in one piece since that's the way the hay is put into the E-Z feeder. A flake of hay full of water is going to be heavy, and if you put two flakes of hay in the feeder, it will be even heavier.
I don't know how you soak your hay, but the way I do it is pretty easy. I put the amount of hay I'm feeding per meal in a muck bucket, those 25 gallon tubs that many people use for toting manure around. Of course, the one I use is not dirty, and is only used for feed. Dump the hay in there, carry the bucket with the dry hay to an area that is both near a hose and in an area where you can drain water, like in a wash stall or outside the barn. Fill the tub with water until the hay is covered, then let it sit. I set the hay to soak after I feed lunch, then right before I feed dinner I drain the water from the bucket by tipping it over to one side, and put the drained soaked hay in the stall to which it belongs. Easy to do since I only have to soak one meal, but if you have to soak every meal then set the hay to soaking after you finish feeding one meal and it will be ready to drain for the next meal. Get a few of those big buckets so that you don't have to worry about not having a bucket ready for hay. If I'm doing something in the barn in the afternoon--after the hay has soaked for a few hours--I drain the water and let the bucket sit on its side until I feed dinner. That way virtually all of the water has had a chance to drain, making the bucket lighter. Remember, soaking hay for only 20 mins will reduce the sugar in the hay by 60%.
I hope this helps.