Jon Sharratt brought in a short piece of a gorgeous, large OD (about 3 inches with .125 walls?) carbon fiber tube he found in the dumpster where he works. It is normally used as five foot long struts to pass simulated loads into a car suspended on an indoor test rig that has five tubes at each corner.

If it isn't too large to mate with a standard bottom bracket this might be ideal for a backbone frame tube. Jon doesn't know who makes these tubes or if they will sell them to "civilians", but he may check into it some more.