I partially solved this problem, for a Glock at least, by wearing a nylon, belt slide holster with a thumb break on my trouser belt. The back pack comes off, the gun comes out of the safe packer, and into the belt slide. The belt slide is flat enough not to interfere with the backpack belt.
It's still a pain though. I think the best solution is a drop holster. I just feel so very commando wearing one...