After about 6 months of testing, I have a solution to the coupler problem. Use the following ideas and pictures at your own risk
It ended up being quite simple. The first test was on wife's Trek mountain bike (where the first 4 pictures came from). Then I did it to my randonneuring bike. My bike has been ridden about 600k with absolutely no movement of the clamps (tested with a micrometer). Feels the same as a non-cut bike. The attached photos are pretty well self explanatory. Nuts and bolts are torqued to specifications and there has been absolutely no movement of the joints, even though the inside surfaces are coated with vaseline as a rust inhibitor. More powder coating will solve that problem.
The two connecting tubes are about 6" long and are cromoly from Aircraft Spruce. Same for the little tubes that the bolts go through. Inside diameter is approximately the same as the outside diameter of the frame tube. Total cost for the connectors was under $20. The two brazed bolts are for a third water cage for anotherwater bottle or an REI cage rocket storage pod for batteries for the lights.
Had the whole thing powdercoated (after my failed paint job). Please disregard the cooler on the front rack. A little test to see the effect of carrying a 9"x9"x6" cube of ice in a cooler as I ride - lots of cold water to drink!
The next step is to build a travel case for it, get/make some cable disconnects, dynamo, etc.