The problem (that can probably be fixed) with strapping a backpack on a frame for me was always two things -
1. It's hard to secure the backpack to the frame in a secure enough way - won't flop around, won't fall off, won't end up in the wheel
2. Most backpacks have a lot of extra straps hanging off them. Like the ends of the adjustment straps on the shoulder straps, ditto if it has any sort of waist thing. And then there's the should straps themselves. Was always worried about them getting caught in the spokes on the back wheel and doing something very bad.

I strapped a jacket to my rear rack once with a bungie cord. Most of the way home one of the sleeves of the jacket worked it's way out somehow and managed to get caught in the wheel somewhere, ripping a hole in the arm of the jacket. Just sayin' it sucked.