I use a holster-style camera case attached by a carabiner to the ice-axe strap of a walking rucksack at the back (clipping in to one of the normal attachments on the holster case) and the shoulder strap of the case over my shoulder and clipped to the opposite chest strap of the rucksack.
This sounds more complicated than it is- imagine wearing a rucksack as normal, then putting on the holster case with the shoulder strap at it's longest extent as you usually would. Then you're just clipping the holster case to the rucksack at two points to stop it sliding around as you walk: one direct to the case itself, and one to the strap.
Although a touch cumbersome when putting the rucksack on or off, I've found this by far the best combination when walking.
The camera ends up sitting about hip level, at the bottom of the rucksack, just behind where my arm naturally swings as I walk. It's nicely stabilised by the multiple attachments so it doesn't bump up and down on my butt as I walk, and it doesn't slide round either as the shoulder strap clipped to the rucksack shoulder strap stops it. Both of those things used to drive me mad on a full day walk in the mountains!
If I want to grab the camera, I can do it one-handed by just opening the holster case and reaching in for the camera, which I have on a hand strap so I can slide my hand into it.
It's not quite as instant as the camera dangling around your neck on a shoulder strap, but it's far quicker than any of the alternatives I've tried, and is it far less encumbering hanging back there out of the way both of walking gait and arm swinging. It works with multiple rucksack options from 25 L daysacks up to the full-tent-and-gear options.
I can put the camera back in the case as well by pulling on the shoulder strap to bring the holster around a bit more to the front- then I can just slide the camera off the hand strap and back into the holster, and do the holster case back up.
I've found it absolutely invaluable for grabbing very transient lighting in the mountains. I don't generally shoot with longer than a 200mm lens though, it might start to get too cumbersome with a big-ass 400mm I guess.