I'm interested to hear the ideas, because I'm having similar water challenges, although more related to volume than temperature. In riding longer distances, I carry two water bottles with HEED in them, so I also like to carry plain water. I am using my Camelbak on my back for now, but it is uncomfortable and I'd like to get away from it. But, I don't know how to carry enough water without it.
Regarding temperature, being in Colorado, I don't have much of an issue with that. Even on 90 degree days, I find that if I start with two bottles that have been refrigerated overnight, they are cool enough to drink for 3-4 hours. After that, I don't know because I have to find a stop-n-rob for refills anyway. In four hours, I usually drink two bottles of HEED and about 30-40 ozs. of Camelbak water. If I ride longer, I have to refill somewhere along the route.
For the Camelbak, I think it would be easy to drink from a mounted position as long as you can mount it with the bottom of the bladder down. The tube connects at the bottom of the bladder, so as long as it is situated so the water is always at the bottom, it doesn't take much effort to drink. I've had mine laying on the floor of my truck before and was still able to take a drink (on the ride home). I have no issues with my Camelbak other than sweating on my back. Keep it clean, hang it to dry when not in use, and it works great. I also use mine to carry keys, Blackberry, wallet, etc. I'm not really comfortable carrying those in my jersey pockets, and my seat bag is full. My daughter has a small one, holds about 30 ozs. or so, and I think it could be mounted. A big one like mine I don't think could be mounted on a bike. It's pretty bulky and would be in the way.