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In short, for true hiking trips, I think you are better off adapting a regular backpack for photo use rather than the other way around.
I agree with this.
Photo-specific packs are great if you don't have to carry a lot of hiking gear, but if you're actually getting out into the backcountry on a multi day trip, 10 times out of 10 I'll take a real hiking pack and just tuck my camera gear in as best I can.
I took a D300, 70-300, 10-20, 50mm f/1.4, and 90mm macro along with a tripod on a 9-day hiking trip in the Andes. I packed it all in my Osprey bag around my clothes, arranged it so the lenses were as accessible as possible, and was grateful every day for such a comfortable hiking pack. I've yet to find a photo pack that carries hiking gear as well or as comfortably as a good dedicated hiking bag.
The pack had a rain cover that kept it dry in most situations, and I had several cheap ponchos that would cover me and the bag for when it really started to come down. Other than that, I had some bags to wrap gear that needed to be put next to anything dirty/wet, and that was about the only concession I made to the photography equipment - protection was provided by unused clothes. I treat my gear very carefully, but the realities of hiking mean that you're going to be very uncomfortable if you try to pad it out much; you just have to accept that things can break.