I've seen a 28.6mm cable clamp. Only one I've ever come across, and I used it to rig a centerpull rear brake on a mixte. It worked great - wish I could find more. I think it was stamped "Diacompe", but I'm not sure. It was stamped with "28.6", and the cable "bump" was offset from what you'd normally expect on a top-tube clamp. Take a look:
