Calton cases are making me a special lightwieght fiberglass case (watertight), and another friend has made nice wide backpack straps that attach to the case. My small rucksack then goes over the shoulders of the guitar case, the straps securly clipped together. I have tested this configuration out with a normal calton case, and the total weight was between 30 and 40 pounds. I went for a 15 mile trek with it and it seemed to work well. The hip support/strap on the rucksack was also long enough to be used and takes some of the wieght off my shoulders. I can of course carry the guitar normally for stretches aswell.
I also share your concerns about the weather, but I'll just have to do my best to prepare for it as best I can and hope I get through it!
Cheers
Joe