I can't see how a backrest could be any use unless you are riding a bike with a 'cruiser' riding position. You'd have to be sitting in a mighty odd way on a GS to even have it make contact with your back. My bet would be that building up strength in your lower back would be more help, or perhaps fiddling with the bar position so that your weight is spread more evenly between your wrists, arse and feet. And maybe a body belt?