"A Garcia is only 12" tall, and the pack is more like 24"+ tall, so if a rolled-up sleeping bag or a rolled-up tarp was to reach to the top, it would have to have 12" of itself sticking up stiffly past the top of the Garcia -- would those materials really have any stiffness at that length?"
Yes and no. It depends on how tightly rolled you make it. The tarp gets rolled fairly tight then wrapped with the guy lines. It holds pretty well considering that it is a fabric, not a structural material. It rolls up to about 25-26" long about 1-1/2" in diameter. It started as a 9x11 tarp. The sleeping bag requires more work sice it IS down. I start rolling the foot box, then up the length, unfolded. It is about 26-28 inches long when I am done. I slip two air ties around it to hold it and suff the bottom in, down around the bear ball. That is usually sufficient. Ranjacket get rolled and stuffed, sleeves in. Again a hair tie holds it. My insulating layers get rolled (about 18" for the shirt and about 10" for the pants and hair tied. The pot, stove, cup and spoon all get put in one pocket with the other carrying the water. It works. Any other clothing(say for cold conditions) gets rolled and added similarly. I tried loose stuffing the bag in the bottom, but that didn't work with the bear ball. It got compressed and fell into the bag. The other thing, I forgot to mention, was compression cords. These will reduce the overall volume and tighten your load. All this stuff sounds like a lot of fussing, but it's fairly simple and keeps things pretty well organized. It might take an extra 3-5 minutes to roll the stuff as oposed to just tossing it in, certanly no harder than in a stuff sack. BTW, this is all done in a liner bag soo things don't get wet. Hmmm, the Garcia's I am familiar with are all about 9" in diameter.
"The external pad pouch only comes up as far as the shoulders, so wouldn't stays inserted in the pad pouch fail to stiffen the extension collar?"
Yes. But by having the stays they act like a "frame", stiffening the whole 22", Note that the extension collar, usually where the flopping occurs, is actually another 10" more, for a total of 32" overfull. The 10" is likely a bit too much. I think I would have shortened the collar to about 6", but that's me, not Glen. Anyway, the normal full height is 22". With everything rolled up below that and extending into the extension collar a 2-4 inches, it does stiffen it. It prevents the looseness at that boundary point, most often the trouble spot responsible for the flopping. Generally I have found you cannot load anything of significant weight in the extension collar area, as is. Note that the Load Lifters attached at about 4-5" will alleviate the flopping, but, again, I have not tried that. My G5 died last year, or I would.