David, I have another possibility you may want to explore. I just got my new Arc Blast and personally will be moving away from a bladder approach, but to your question…
The Arc Blast has a feature that isn't very common in the world of backpacks, a trampoline suspension that leaves a cavity between your back and the back of the pack. This provides airflow etc, but one of the criticisms of this design is that it moves the pack weight out away from your body, which affects center of gravity, centrifugal force when you turn/spin, and overall carry such that it feels "heavier" than the same weight closer in.
My old pack also has this trampoline suspension approach, and it was sold at the time as having the ability to hang the water bladder inside the cavity between the trampoline and the back of the pack. This moves your densest, and often heaviest, item in towards the spine and stabilizes the carry, negating some of the negative weight-distribution effects of the trampoline design. You could consider this approach with the Arc Blast as well.
This will cut down on airflow across the back, obviously. With my old pack, generally the bladder hung in the deepest part of that cavity, which on me was shoulder blades to middle back. When the bladder was full it laid against my spine, so no ventilation there at all, but it was a much narrower point of contact (because of the bulge of the bladder), so still allowed airflow to the sides of my back to the left and right of the contact area along the spine, as well as straight across above and below the bladder. And as I drained the bladder it flattened and loosened away from my spine and allowed a lot more airflow. So all in all I thought it worked pretty well.
I haven't tried it with my Arc Blast yet, but I have noticed a few things that will impact. First, my old pack was specifically designed for hanging the bladder in the cavity and has a loop at the top inside of the cavity to hang the bladder from. With the Arc Blast to hang the bladder you could just tie a bit of cord to the haul loop and hang the bladder from that, at whatever height you want. Or, if your pack is still in production you could ask Joe to sew a small loop for you, somewhere under the haul loop (you should measure your bladder first so that the loop allows the bladder to hang at the height you want).
Compared to my old pack the Arc Blast has a smaller trampoline and thus less of a gap behind your back. So the cavity will be tighter for your bladder, especially if you get the lumbar pad. But another thing is that the frame on my old pack was an hourglass shape, so in the center the frame was narrower, more in along the sides of the back, and that held in the load from the pack better and held that cavity open. The Arc has a straight frame and thus doesn't hold back the pack load as well. Depending on your load and how you pack it, you are more susceptible to the pack load bulging out into that cavity, which would press the bladder more firmly up against your spine. You could help this by packing your pad against the inside back of the pack, and/or you could tie some cord taut across the vertical frame pieces, in between the two horizontal pieces, which could hold back the load some.
And, of course, another factor is how much "arc" you decide you like. On my old pack the arc is fixed (not adjustable) and is out about 2.5" in the center, which happens to be the same amount Joe suggests for the Arc Blast.
Just another avenue to explore.