Liquid cooling is a great concept, because you only have to cool your torso, and you only have to add a total of ten pounds to your back. (5 for the pump, mini-cooler, and power source, 5 for the ice and water) and you should have the hoses filled with antifreeze rather than water so you can actually let it sit unused, and only start the pump for five minutes or so at a time when you need to shed a few degrees of excess heat.
It can also be used with hot water in the cooler for those cold northern winter troops.
Essentially, you run tubes up and down your torso (Siphon effect counteracts the "uphill pump" problem) and then run a few loops of the tube through the inside of the cooler. When the fluid goes through the cooler it's either shedding your excess body heat into ice, or absorbing heat from your hot water. You really don't need much, and the cooling effect cuts more than enough heat fatigue to overbalance weight fatigue when used properly.
More weight can be cut if you made it a plug-in system and only used it to refresh yourself for a few minutes every hour or so by plugging into a table power source, going to your hotel room, or even using a car battery, like the racing pros do (CoolShirt under their racing suits. Those guys have it far worse than we do, thermally)
Call it a DIY CoolShirt. You could get a REAL CoolShirt system, but that's prohibitively expensive.
The main thing to remember is that you don't leave it turned on all the time. That's a ticket to hypothermia.
