Very cool idea, good to see someone 'thinking out of the bag', so to speak. Can't help but remind me of my old REI external frame pack with the sleeping bag and tent stuck under the main storage sack of the pack.
How does this carry with the can almost empty of food? Seems like it might be a touch top heavy at that point, have you found the way the weight distributes over a week noteworthy? that's the only real downside I can see with this method.
If I were a cottage gear guy I'd take a close look at this, it really does seem to solve a big issue with UL/SUL packs, at very little cost beyond the possible top heavy part of the weight distribution, but then again, ul loads aren't that high in the first place. I use a Lowe zepton true 50 liter pack (ie, internal bag volume is really 50 liters), and bv500 fits fine across the top, inside the pack that is, horizontally, but that's not a sul pack, just a quite light one at 2.5 pounds or so.