Backpacks generally hang way too low. It would interfere with the front wheel unless you took specific measures to prevent that. Maybe if you strapped it horizontally it might work. Might be inconvenient removing it.
I hang a small backpack off a dowel cable-tied to the saddle rails. Very cheap but effective. This method was invented by Russ Roka of pathlesspedaled and also used by others here. This method does not interfere with the fold, although I do remove the pack when folding fully to prevent the chain from soiling the pack.
Hmm dowel doesn't seem to be working too well. The length of my backpack combined with saddle's fore-aft position is causing a few inches of heel strike. Any ideas on spacing it farther away from the seat post, besides moving my saddle (which gives pretty comfortable pedaling atm)?
Ah yes, the heelstrike. I forgot about that, my pack is small so I just get away with it. I did mess around at one stage with a roll made from light packing foam strapped to the seatpost to space the pack away from the seatpost clamp on my Moulton... it didn't work very well... it also interferes with the seatpost sliding down when folding the Brommie... it is a tough problem to solve, basically you need some form of T-shaped spacer that the pack hangs against. But it needs to be removable for when the bike is to be folded.
Perhaps something can be attached to the pack instead and just quickly velcro'ed to the seatpost to keep it correct while riding. I gave up and strapped some protective foam on the Moulton; the other bikes have long seatposts so this is not a problem.
Perhaps you can find a solution involving the wheels on the rear triangle that the pack rests against when riding.