There is no one size fits all solution. I have wrestled with this for some time having tried chest harnesses and slides with not much success. I have also found that if I can't reach my DSLR with my pack on, I will forgo taking a number of pictures.

For backpacking, I carry my Sony A55 with an 18-200mm lens, polarizer filter and memory cards. I will sometimes carry a DSLR Gorillapod. The 17-50 f2.8 and 70-210 f4 stay home as the weight benefit ratio doesn't work for me.

Right now I am using a Tamrac case that works very well with the way I have connected it to my pack. But each case and pack are different. A picture of my pack with a prior case (in red) that worked pretty well is below.

It sounds like you want to put quite a bit of weight in camera equipment on a 45L bag. I am not sure how this will work, especially if you want access to your camera (which is much bigger than mine) and an additional lens.