Nobody mentioned the Ruger Single Six or Single 10. If I were looking for a backpack gun, those would be high on my list. They are a bit pricier than the 22/45, though.
I have the Ruger MK II Government Model, 5 inch. Was the sub caliber pistol when Army used the 1911. Heavy, accurate, and reliable. Take down like any other Ruger MK. If you are in a BOB situation, you are most likely not thinking about take down and cleaning.
Reliable with any .22 ammo you can find. Durable. Compact. I have the 4 inch barrel, and have made 50 yard shots with mine, not rifle like accuracy but will hit a ground hog sized varmint.
I would consider the SP101 also, but they weigh a little more, if that is a concern.