That's a neat idea!! That way the backpack stays vertical. Cool!
the problem I'd have with that is that I have a hard time getting my leg over the seat as it is. With the backpack like that, I wouldn't be able to get on my bike.
When I was younger, I could hop on my bike and throw my leg way up into the air after I was already on the pedals. Not anymore! LOL Anything above seat level and my bike becomes unridable.
Thanks for the cool thought, though!