I have been experimenting with air injection in my back. I use an home made version of a Ventisit and with some tubes I channel air inside it to refresh the back. I have many messenger bags, many backpacks. They were all not OK. I own some recumbent bikes (which have a terrible back sweat problem, unless you use those ventilated seat pads). Ventilating the back works. It can potentially work without air scoops, but not in the middle of the summer. I live down here in Tierra Caliente in Italy and without that technique I would have kept only the bikes with rack and panniers.

Basically it is like adding tubes to the Blue Lug Messenger Pad:
http://bluelug.com/products/detail.php?product_id=11721

http://bluelug.com/