I posted way back last summer about altering a daypack so it would have a diagonal zipper across the back, the part that lays on your back. I have a smaller leather pack that has this feature and I love it.
It worked really well, but I honestly never felt I was in danger of being robbed (England, Scotland, Ireland, Netherlands, 6 weeks, last September and October). The Eagle Creek daypack held up OK, after I sewed up one of the straps that was coming apart at the seam across the top.
I hope this helps someone, or can push manufacturers to use this design.