I have never seen them in person, but I noticed the other day on one of my favorite shopping websites (Valley Vet Equine) that Circle Y now offers 3 different types of pads for bridging/swayback. Each one made to fit a different shaped back. The swayback type even comes in two different thicknesses.
I think that is an awesome idea. It's about time someone put some thought into the issue of helping horses with less-than-typical backs ride more comfortably.