Thanks for those suggestions! That heimplant bag definitely seems like a good possibility. The only thing that's missing is a place to put your water, but I guess you could just hang it off one of the webbing pieces.
I'm actually really strongly considering just getting a Synapse 19 and doing the shoulder strap attachment that was posted in the pictures earlier. The bag has everything I'm looking for in a backpack and it has viable attachment points for a strap. I don't plan to carry it much like that, just while I'm in the airport or have the Aeronaut as a backpack getting to my destination.