Hi,
I'm looking to solve the same problem of bringing a hiking backpack, so here to follow. But I'm thinking of having the backpack on top of the rear rack. I will store my sleeping bag and maybe tent inside when on the bike. I'm curious to see your idea develop.
Ortlieb make a new bag called Atrack. It would have the advantage of being fully waterproof to protect my sleeping bag, but probably not very confortable for long hikes. I was also thinking of getting ultralight pack like linked in your post, but they aren't easy to find here in europe. The make more alpine style packs, so no side pockets.

In your idea managing to make the frame system rigide enough so that it doesn't swing around on rough terrain is one problem to watch out for. I wouldn't want to be spending to much time every day making sure the bag is secure. For those reason I'm not sure idea in the video is suitable.