Oh, man. I have no idea. I have a friend who loves physiology and horses, so she always talks about how wings would have to be enormous. She did an illustration of a Pegasus that basically had a double shoulder blade mechanism. This www.shapeways.com/product/YE8F…
doesn't have that, but I do like the idea of it having a very large keelbone (wishbone) to anchor super powerful wings.
James Gurney suggested Skybax riders (people riding giant Quetzalcoatlys) laid flat to avoid interfering with their mount's wings. Plausible, but definitely not dynamic (you couldn't lean over and throw stuff riding a Skybax.) You can Google Skybax for his illustrations.
I know a lot about wing mechanisms and how flight evolved, but riding a flying thing pushes past the idea of "How much room for the rider?" and into "How does the rider not interfere with flight?" Evolved flight is so delicate.
But the good news is very few people know how flight works anyway. I'm with you that a rider's knees being behind the wing would be better (it would only interfere with the mount's ability to folds its wings as opposed to flap) and maybe even further back than that (no touching feathers at all, which gets into the lying down position James Gurney suggested.)