If I've donned a fully loaded backpack, I suppose the bandolier with the strap under the pack straps and the bandolier draped across my chest is going to be about ideal for transport purposes and still have a chance at quick deployment. Hip holsters won't be functional if I am wearing a padded hip belt on the pack, which I'd do with any pack weighing more than about 10 lbs. A crossdraw might end up with the gun hanging up on the shoulder belt on the draw. There may be a way to custom rig a crossdraw using the shoulder belts of the backpack.

If you are surprised on the trail and your gun ain't already drawn, then it's likely the attacker, be it man or beast, has the drop on you and you aren't going to get a shot off unless the gun is already in hand. Like Chris said.