The pack I ended up choosing for the JMT hike was an '07 SMD Comet.
I could pack it the same way I did the Mariposa with a Gossamer Gear Nightlight pad cut to 36" and unrolled INSIDE the pack. The edges of the pad just barely touching each other. The Comet has a good bit more girth to it than the Mariposa.
The other factors that made me choose the Comet were the bendable aluminum stays because of their customizable fit, the "load lifter" straps because they pulled the top of the pack closer to my back and the way the hipbelt wrapped around my pelvis. Although these features added weight to the pack they made the percieved weight of the pack less, due to the improved fit.
Like Will I would say replacing the straight carbon fiber stays with curved ones, whatever the material, would be a big leap forward in the Mariposa's design.
Ditto on the velcroed shoulder and hip belt.
I used spare clothing as shoulder/hipbelt padding in my earliest GVP Gear packs but, after the novelty wore off, went back to using foam padding. Except with a thinner 1/4" thickness in the shoulder straps. All that daily ripping open, stuffing and unstuffing and restuffing became a real pain in the butt.
I like the Mariposa best as a frameless ultralite pack for 2 or 3 day winter camping where my gear is bulkier but the weight doesn't exceed 15 or so pounds.
The Mariposa is my wife's favorite pack, with a Prolite 4 short unrolled in the interior, the stays installed and a Nightlight sit pad in the exterior pad sleeve.
She really likes the extra volume so she can load her sleeping bag and any other high loft garmets unstuffed.
She doesn't notice the pack riding too far from her back due to the straight stays and when I pointed this out to her replied "NO! You are not buying me another pack!"
Sometimes stuff only bothers people after they learn that its supposed to.