I received the 3/4" x .120 wall stainless tubing in the mail yesterday, and made some progress today on the rack / pannier mount system.
Here is what it looks like now:
The larger diameter (3/4") tubing coming out towards the sides of the bike will be one of the main supports for the panniers. This tube is supported pretty directly by the rear-most mount points of the frame (which you can see I beefed up earlier with triangulation in the sub-frame), so most of the weight on the rear rack, and much of the weight of the panniers, will go straight down into these points. There will be two other points of attachment to the bike as well. You can also see the diagonal tubing that I added, which goes from the very rear of the top rack down into this same area just discussed, the mount-points at the rear of the bike frame. I made a provision to run the turn signal wires within these diagonal tubes, which you can see mocked up (the green and orange wires disappearing into the tube). Purpose-specific tabs on the rear rack carry not only the turn signals, but also the rear fender.