Thanks all
MSN, I thought of that for my seatpost bag too, it looks like an old school front rack hoop that goes over the wheel to stabilize the bags. I think it would do the job for a seatpost bag. I know Portland Design Works makes a similar product called the Bindle rack.
Jefnvk, look at the Carradice website. They have the quick release system but also what they call a retro rack that permanently attaches to the bag and then the bag/rack can be taken off the bike. What bothers me a bit about both designs is that all the weight is still pivoting off off the rails and seat post. It's probably no big deal but the mechanical side of me doesn't like the stresses that would create on bumpy rides. All you need on a tour is a big drop and your seat or post breaking away with the bag.
No Control, I've seen pics of a bikepacking set up with Carradice bags front and rear. It looked like they had something like a Nelson or Barley up front and a Camper on the back. Kinda cool retro.