Well, when I used to do alarms when I was younger and in big cities and bad areas, I carried a Colt 1911 at the time ON me, and I kept an Uzi in my toolbox. I kept an AK behind the seat of my work truck and we paid a guy minimum wage to ride along and sit in it when I had to do jobs like that. Unconventional, but it worked.
Now I just IWB in a Milt Sparks VM2 with a G29 or G27. My wife, she purse carries because she kind of has to. It isn't ideal, but like they say, better than nothing.
I like the idea of the modified backpack so you can reach behind you, that sounds pretty sweet.
If you are a worker that wears a workbelt, this is easy. Just get an industrial type nylon or leather rectangular "holster", something for tools or whatever, make it if you have to, then have a cheap holster installed or glued inside to cover the trigger. Presto, you have a completely concealed and safe pistol on a workbelt. I've done this, and you cannot tell at all. That Glock 17 looked right at home with a Buttset, screwbag, tookit, etc.