Update, in case anyone is interested...
I ended up going with the Super C Audax bag. Saw some posts for DIY bagman equivalent racks and decided to try making a (sort of) quick release version. Looks ugly but seems to work OK so far. I used soft steel rod to form the support and automotive hose clamps to attach it to the seat rails. Made a bracket to bolt to the B17 loops to hang the bag using a straight section of rod through some eyebolts attached to the dowel. So far it has held up on a couple of brevets as well as some bumpy fire road rides. But I may end up getting the Carradice Classic rack to make it easier to move the bag between bikes.
