Your chemical heat pack pocket is an interesting idea. You may also want to consider that in a hiking scenario, the wearer will probably have on at least a day pack if not a hydration pack so the pressure could create a hot spot on the wearers back. You might consider having internal breast pockets to serve the same purpose. This would function better to keep the body core warm as well as make the heat packs accessible for hands to be warmed if needed and to be replaced more easily if your in the cold that long.

If the natural design of the back allows for a pocket it would be great to keep it as an added feature but in function I'm not sure most people will use the pocket since they would have to take the garment off to get stuff out of the back. I only use my back pants pocket for a wallet and that's only when I don't have a jacket with a internal breast pocket. I just prefer carrying stuff I use frequently on my front where I can get to them easily and keep an eye on them.

Once i put something on I tend to want to leave it on if I can rather than carry it in my hand, and I'm not a fan of having to put things on and take them off to regulate my body temp thru-out the day. I'd rather be a little cold for a while or a bit warm knowing I'll adjust once I'm back in the predominate temperature

Please don't think I'm slamming your idea, I think it's got a lot of potential and I'm just throwing a bunch of ideas out there to be used or not used when you're ready.