This kind of goes back to the pros and cons of the late Colin Fletcher's Trailwise external pack frame that had an enormous mesh back band.
Many internal frame packs are touted to have ventilated back panels, but only a few have suspended mesh panels, and of those, the only ones over 2000 c.i. that I could find with the mesh connected along its entire perimeter were the Ospreys.
This is important to me, because I find that it is only when the entire perimeter of the mesh is stretched and connected to the pack that it molds to my back and flexes well as I move. It is also nice to avoid creating an inaccessible dream spot for biting insects to get at your back.
Otherwise, why not just use an inverted U-shaped tubular frame with curved crossbars, stretch a wide mesh band and a waist belt harness horizontally around it, and attach whatever pack you like that fits. And with tough caps over the bottoms of the legs, the pack bottom is protected from a lot of abrasion.
There was a pack of this type made by Alpine Designs that used an ABS hour-glass shaped tubular frame, with the mesh connected all around. This is what I use, with all the fabric but the backband replaced by 4 oz. (total) Spectra gridstop.
It was because this pack was so comfortable to carry doing trail work, with heavy equipment thrown into it and the waist belt removed, that I converted it for regular BP, adding an Osprey moldable hipbelt. The pack is about 3000 c.i. and weighs just over 3 lbs with the fairly heavy older style Osprey belt.
Ray's post suggests that Osprey has reduced the distance between the mesh back and the pack back, but it does not sound like that from Joe Clement's post. Osprey probably uses a lot of tension and distance to avoid complaints about things poking through into customers' backs (there may be no heavy foam or frame sheet). Unfortunately, this not only reduces pack space; but as also noted on this thread, pushes the center of gravity of the load backward, cancelling out a lot of the comfort gained from the suspended mesh back panel.
In remodelling my own, and making another pack with an aluminum tube hour-glass frame and suspended mesh back panel, I learned that the ABS works better for loads under 30 lbs (total) because it is more flexible, and that the dimensions in the design must be extremely precise for the panel to come out stretched tight. Sort of like a geodesic tent design. So I imagine it is a bit of a challenge for the manufacturers to build to this design, but if Alpine Designs could do it, it can be done today.
In the meantime, when my home-made item wears out, I will buy an Osprey, and maybe modify it a little to move the center of gravity forward if they haven't done that already. With this type of design, while I'm sure my back sweats, I have never felt it. It really is a lot more comfortable, and I wouldn't settle for anything less were I to go trekking in the tropics (fat chance).