Blackhawk Serpa
I know this thread is a little old, but I just came upon it. When I was backpacking in Colorado last month, I tried my new Blackhawk Serpa holster with my Glock 20. This was by far the most comfortable way I've ever carried a gun while wearing a backpack. I used the paddle attachment and put the paddle over the hip belt, but not into my pants. At least with the pack I was using the paddle gripped very tight; I could take the pack off without worrying that the holster was going to fall off the belt. I was completely unaware of the presence of the gun, so much so that I had to reach down and feel it from time to time.
But it's then a snap to take the holster off the hip belt and put it onto your pants for those times when you do not have your pack on. The Serpa also holds the gun in the holster very securely. So, at least for me, this was a winning combination. I wish there were an equivalent method for carrying my Freedom Arms .454 Casull, but there is no Serpa for that gun.