Some packs allow a few options. My Granite Gear pack has webbing on the outside of the waist belt that is compatible with MOLLE so if you've got a MOLLE holster option, that might work.
What I'd really like to see is a shoulder holster body that attaches to the pack straps via grim-loc or similar connectors so it becomes part of the pack. I haven't found anyone making quite the right rig yet.
