I have at times taken heat for the huntseat position. I tend to hollow my back on the flat much like we used to over jumps. (aside: I don't jump any more - promised Kate I wouldn't).
There seems to be, just as you note, (and from those not proficient in huntseat/forward seat/caprilli) that the hollow back makes one stiff. I never had a single student or eventing team member that was stiff.
Those attempting hunt seat for the first time do experience stiffness while learning. What new position on the horse, including first time riders doesn't result in some stiffness that eases with practice?
The accusation of stiffness when first trying huntseat is akin to accusing someone of having bad hands that just started riding. Huntseat needs to be taught. And practiced with drills like any other "seat."
I've ridden many "seats," because I could, and practiced them so that I was proficient. Even Western riding at one time looked to some seat styling that the researcher/trainer/teacher modeled after Caprilli. A forward seat. He called it "Balanced Seat." Monty Foreman, who many have never heard of but at one time was the leading author/clinician in the country.
He was working reining stockhorses using a forward seat style. Really.
Hard to believe, but the photos of his and his students sliding stops, rollback, etc. with a forward seat showed huge talent and effectiveness.
They stood as though to jump, came up off the saddle, heels down, knees and hip angled, and absorbed the shock of the front end so that they didn't load it (though it was claimed they did by those who could not accept the change), and they got OFF the hindquarters so the horse could bring them up under the belly without the extra weight to the rear we saw then, and sadly see now.
In western reining work today hock and stifle injuries and wear are an occupational hazard for the horses. And you likely have seen the current popular, sit down, shove your feet forward, tuck your butt under style that's dominant.
When those that practice this - all currently - describe the mechanics it's as though they build the horse anew, and defy both basic physics and biomechanics.
Horses have to work like never before.
If you can get ahold of any of the books of Monte(sp) Foreman take a look at pics of him sliding horses. The current idea that a horse can't balance with the rider shifting weight to stirrups and off the bum and saddle is laughable when one looks at the top jumpers.
I couldn't afford one of Forema's Balanced Ride saddles back in those days, but had a saddle maker that owed me big time and he built one on a tree I provided. Flat seat, pommel to cantle. Leathers hung a bit forward so as one rose it was more like a jump saddle feeling.
I started training green colts with it, and boy oh boy did it ever work.
No one had ever, up to that time, about 1965, seen knee support rolls on a western saddle before, but I had them built in, and found out a few years later cutting horse people had started to order saddles built with them. I suspect my friend had started the trend as he was a cutting horse man as well as a saddle maker.
All that exploration in the past has made me attentive to the current efforts to move ahead with saddle design and especially with treeless.
Soon we will see more partially treed, or partially treeless, saddles out there and materials with properties that better fit and move with horse and rider while providing the support and variable degress of stiffness in reaction to movement, weight, change of balance, inertial forces etc.
We have strange polymers now that if you hit them hard you'll hurt your hand but of you move slowly into them you can bury your hand painlessly as though it's a cushion - and all degrees of resistance along the spectrum from very hard to very soft impact response.
Exciting changes taking place, and I hope some that will accommodate the rider with compromises of various kinds. As I age I certainly am finding more of them in my own "seat," and ways of riding.
Best wishes, Donald, Altea, and Bonnie Cupcake
Love is Trust, trust is All
So say Don, Altea, and Bonnie the Wonder Filly.