The first version worked pretty well. It did have a few problems:
My second version basically attaches the same fittings to an aluminum bar with pop rivits. The bar is then clamped to the saddle rails in a way inspired by the Carradice Saddlebag Clamp.
Although heavier and more complecated to make, it seems to overcome all of the problems with my first design. I have them on a couple of bikes and can move the bag easily between bikes and take it off the bike to go shopping.
This design fits both Brooks Pro and a Swift. I'll try to post a drawing soon.
I don't know how it will hold up, but I think it will do better than the plastic quick releases held on with ty-wraps that I was using.
Some smaller pins would be nice, and they need some attachment so you don't loose them, but it looks like it should work. My test ride was with a "dummy load" of a 5 lb. bag of flour. It did fine.