I've successfully used a chair kit for a Thermarest to do the same thing with my Granite Gear Virga pack. I leave the stays in their sleeves, and the Thermarest in the chair kit, fold it in half (width-wise, where the back meets the seat), then roll the stays in toward the middle to get the desired width. Then I insert it along the back panel of the pack, "lock" it in place by stuffing my sleeping bag (inside a bivy sack) into the bottom of the pack, and finish loading it. If necessary for comfort or rgidity, I can inflate Thermarest lightly. I've tried it with the stays toward the inside of the pack, and along the back; it doesn't seem to make a lot of difference in comfort.
The Virga is rated as carrying a 20-pound load comfortably with just a pad for a frame; this seems true, based on my experience. The addition of the stays moves the comfort level up toward 25 pounds.
The Prolite 3 Thermarest seems to work best - anything thicker has too much bulk to fold well, and is rather unwieldy to pack around.
Now, the obvious question: how can you call a Thermarest, let alone one with a chair kit, "ultralite"? Well, if you're out there on the bleeding edge with sub-5 pound loads, you obviously can't. However, for those of us merely on the bruising edge, it's a relative thing. For example, I carried a 2-pound, 2-ounce Vapor Trail pack. Changing that to a Virga with a chair kit (11 ounces) plus the Prolite 3 (2 ounces more than the Z-Lite it replaced) saved me nearly a quarter pound in weight, and gave me the luxury of a soft bed plus a chair kit, and an extra 5 pounds of capacity. (Of course, almost one of those pounds was used up by the pad and chair - like I said, it's all about trade-offs.)
For whatever it's worth, while I've used this system with good results, I mostly stay with the Z-rest as my frame and keep my loads at or under 20 pounds. Personal preference, mostly.