What about a second, smaller bag that you wear backwards on your front of if you can manage it, a chest rig?
Maybe something smaller that you can put some of the gear you would normally use for those "off the cuff" moments that you need to snap some shots while you are huffing it out to your destination, say one of the DSLR bodies with a lens mounted, a second lens and some of your filters and smaller accessories?
It would split the weight load to both your front and back, allow you to place your larger pack in the overhead and the smaller one closer to you in the under the seat spot on the plane.
I started doing this and it both made me more stable while hiking over rough terrain and reduced the wear and tear on my lower back.
I uses this case for my front, smaller pack that actualy works as a chest rig: http://www.walmart.com/ip/Case-Logic-SLR-Camera-Holster/10929537
I took off the carry strap and put that in my main pack to use later if I am just going on a short walk from base camp to get some morning shots. I attached d-rings to the carry strap mount points and two points on the straps of my main pack and run the chest stabilizer strap from my main pack through the belt loops on the back of the camera case. It effectively makes a rather stable chest rig that's comfortable and insanely useful. This can hold my Olympus E-300 volt very nicely and the side pockets have enough room for me to carry it's spare battery, extra memory cards, some filters, etc.
I also use a few different lens cases that I can attach to the straps of my main pack so I can get to say a high zoom lens for my E-300 Volt while I am hiking. Makes things so much easier to get those quick shots and again, balances my load between my back and front.
Just a thought.