What's the best way to wash the pack part of a hydration pack (not the resevoir/hose)? Is it just like a back pack: cold water and a little detergent with air/tumble dry low?
Here's what I found from Dakine (brand I have), guess I should've trusted Google first!
As others have mentioned and you've discovered, the safest way to clean it is to simply follow the manufacturer's instructions.
My girlfriend and I both have Camelbaks. Their website says: "DO NOT wash your pack in a washing machine." I've done this with both our bags a whole lot of times and have never had a problem. I've also put a bunch of backpacks through the washing machine that weren't supposed to go there, never with a problem. (Can you tell I'm lazy?) And a hydration pack is basically just a backpack with a special compartment for the bladder, and maybe a bit of extra padding or insulation.
I would, however, recommend hang drying them, no matter how you choose to wash it. Dryers can get hot enough to melt the nylon that these kinds of bags are made out of. Low is probably safe, but why risk it? They easily dry overnight, even indoors.