Barry, I think you are right. The modern seat for jumping is great for jumping, but only if you first develop a good, independent seat. For that, a long leg helps a lot.
I've been trying to teach myself to ride from reading, and most of the books were written by dressage riders. I'd try to move my hip bone here, put my leg there, curl my lip in a snarl like this, and...it was like the scene in "The Paleface" (http://www.amazon.com/Paleface-Bob-Hope/dp/B00005UMF6/ref=sr_1_1?s=dvd&ie=UTF8&qid=1299531612&sr=1-1) where Bob Hope tries to follow all the advice of gunfighters and gets everything screwed up.
In December, I gave up. I ordered an Australian style saddle and decided I was going to just have fun. And if Mia wasn't thrilled, she could suck it up because I was feeding her and she didn't have to be happy all the time. And to heck with all the dressagy advice, and lengthen my stirrups and try to have fun.
And an odd thing happened. When I stopped trying to put my leg somewhere, and couldn't prop it up on a short stirrup (no, I can't blame that on dressage), gravity pulled it underneath me. And if your leg is under you, your back has to straighten because it feels weird otherwise. And since I wasn't worrying about my horse's reaction, I stopped looking at her head - so my head came up and my arms came back, and...
...and I started looking more...more dressagy? Mind you, any beginner dressage rider could still find lots to critique, but one passing by would no longer slam on the brakes, jump out of the car, and shout, "Who is that deformed man, and why is he clinging to his horse's neck?" When I gave up, I started to succeed. And when I stopped worrying about Mia, Mia started to relax as well. Her tension - some of it - was MY tension, reflected.
That was when I realized why young kids learn to ride better than adults do. They have fun. And while they have fun, their bodies adjust. That doesn't mean we can't have instruction, but we can't solve 12 things at once.
I like the instruction my daughter-in-law is getting. About twice an hour, the instructor says "Toes up", or "look ahead", and that's it. The rest of the time she is being taught the cues for stopping a horse, or slowing the trot, or navigating obstacles or cones or whatever...and after 4 lessons, she sat the trot almost the entire hour. No one told her it was hard. She was focused on something else, and lesson 5 was done almost entirely at the trot, and sitting, and she was smiling and having fun.
I can look and see she is leaning back a little, and her feet are a bit forward, but I spent TWO YEARS working to get where she is at on lesson 5. And she didn't work! I've spent so much time making riding hard, when it was supposed to be fun. To sit the trot, accept the horse's movement rather than fight it. Enjoy your horse. Work on something together. If you bounce a bit, so what? It won't last forever. Your body will figure it out. Your mind cannot.