But, that's still feasible. I could just have half the pad acting as an internal frame, and the other half strapped outside for easy access for sitting. But, I'm still left with the issue of how to reattach those pieces for sleeping -- does velcro or duct tape adhere adequately to the irregular shape of a ridgerest, zlite, or night-lite?
Two pads would work, I think. Velcro doesn't work all that well unless you are a still sleeper. It also gets dirty fairly easily, needing a good cleaning to restore it every 5-6 days. Threads, lint, duff, all stuck in it. Oils can saturate it, even water drying times are poor in comparison to a CCF pad. (The NightLite is waterproof as is the ZLite.)Off hand, I would say it would not be my first choice. Especially near the middle. If you make a two and three section pad, somewhere near the middle will be the velcro... not a good idea, but, I toss and turn all night due to an old back injury.
Duct tape will last about 5 years, then it sort-of stiffens and loosens. The Nightlight pads are really only good for that long, then they tend to flatten out. Generally it works pretty well. The NightLight pads are about 3/4". The bumps add a bit of insulation when placed up, since the bag will loft into them, if not collapsed with your weight, or, if partially collapsed. With them down, the pad is slightly cooler. This applies to the "pack frame" as well.
I found that the bumps were quite important to stiffness. The ZLite did not work as well, collapsing after a few miles. But, there was only about 1" of pad, so this could also account for the collapse. Two pieces of interlocking bumps measures about the same as the ZLite, but is *much* stiffer when interlocked. Insure you do that, IFF you decide to make one, though it means wasting a couple inches.
Well, as far as pulling away from your body, as I said, this is a "personal preference" deal with 20-25 pounds of pack. With 30, it is noticable to me. But the old G5 is only rated to 15pounds without the pad. Sort of a trade off for the extra food(and weight) needed on longer trips. It works far better than rolling a pad inside the pack and filling it with gear.
For the old Jam, I made one in a rectangular "box" shape to stiffen the bloody thing. I used this a couple times but gave the pack away to a friend after three trips. It worked real well, but I missed the seat.
Generally, I have three differnt NeoAirs, an Inertia, and several different CCF pads and self inflatables. None of the inflatables or self inflatables have nearly the same stiffness, even partially inflated and with tight pack compression. They will crunch up after a few miles. The Jam was particularly bad about weights over 18pounds. The 1/4" CCF is OK *only* on forest duff where I can burrow it in to my body shape. The 1/8" CCF is a cross between a ground cloth and a light pad, it doesn't seem comfortable to me without a LOT of ground prep.
The CCF pads work fairly well for all soft surface sleeping. On hard surfaces (lean-to's, flatter rocks, compacted "used" camp sites,) I prefer the NeoAir. For me, the Klymit Inertia pads do not work as well as the NightLite, NeoAir or Prolite. It seems to work about as well as my old military issue 1/2" CCF pad.
BTW, I am retired, actually an old man, so the wife tells me. Funny, it still feels like I am 25... I have an old back injury, broke my hip, broken arm, wrist, and shoulder. All feel it when I get up in the morning. I *need* a good pad so I am a bit more carefull about them than most, I guess.