I don't think it was by any means just Konas that needed 30mm clamps Rich. Tange seat tubes and I think most others at that time had their butting at the top external, so that the inside of the tube was of constant diameter. So a 1.2/0.6/0.9 x 28.6 tube means (as far as I'm aware) that the 0.9-0.6=0.3 butt is at the bottom and the 1.2-0.6=0.6 butt at the top makes the external diameter 28.6+2x0.6=29.8. And for the same reason, you need a 33.0 clamp for most modern 31.8 steel seat tubes.
You can only use a 28.6 clamp on a tube where the butting is internal, but I don't think that kind of tube is as good for mtb, as the post is only being supported by the few inches at the top where the butt is (unless it is only single-butted and there is no butt at the top, which is even worse).
Are you sure about that NeilG? I've known Hei Hei owners to say that and then fiddle around with a torch etc and say 'oh yes!' I thought it was that you need a sleeve in a ti frame because if you put a ti post in a ti seat tube without something separating them, they can get very profoundly attached to one another.