You said shoes aren't an option, but that's what I do with my thin soled horse.
He gets a correct trim, NB shoes, and pads. He only wears pads when we're riding, mostly because my arena at home is rocky. He does wear shoes year round though. His feet look better since I bought him, partly due to a change in his diet. He's on a low starch ration balancer, flax, MSM, free choice grass hay, and 2-4 pounds of alfalfa. He has much better looking hooves now, but I still keep him shod. The NB shoes have really done him good.