You can also throw that thingie in the freezer and start off with a solid block of ice which will still take hours to melt even in extreme temperatures. The pack insulation will both slow the process and result in your back being cool instead of cold. In any case, the total weight is variable is - nothing says you NEED to always completely fill it every time.

Hydration isn't always the issue - particularly during a relatively short few hours. Sometimes you just need to cool down, and a washcloth loaded with cold water can be a lot more effective than all the perspiration in the world.