Oil Pan, Great build, nice high quality work. I can assure you this will work for all the reasons you posted above and from personal experience. I am using/repurposing a medical device called the Polar Care Cube that I bought after an injury. It uses about a gallon of ice water circulated through insulated tubes to a 9"x12" pad designed to reduce swelling on an injured knee. This is my second summer of testing and I got 60 mpg highway tanks in summer driving in 2014 with this device and almost no use of A/C. I power it with a small inverter plugged into a lighter socket.
I have found that with this small pad velcro strapped to the headrest and the pad at the base of my neck, I am comfortable into the mid 90s with fresh ice for about two hours. I dress for heat (shorts, light t-shirt and flip-flops) have tinted windows all around and use a cooling towel I picked up at Lowes. On long downhills where I don't want to exceed the speed limit I run full A/C in DFCO which adds a little more engine braking to the diesel engine and uses no fuel in the VW. At slow speeds in town I open the windows to flush out some cabin heat.
With your big cooler and pads I am confident you will be comfortable and be able to keep a passenger cool as well. Mine is too small to use on anything but solo trips so my mileage suffers when I have a passenger.
http://www.buymedtech.com/breg-polar...tain_notices=Y
(I just noticed that this devise is supposed to be prescribed by a physician.)