They have done experiments using RF over air, not in copper. However I do nit see the difference, you are just shining em fields down the inside of some copper when it comes down to it, instead of air.
One experiment that I saw simply unvolved an ordinary parabolic dish aerial that had been butchered and a cut was made radially our from the centre while the outer edge was pulled forward, so the whole thing had a twist, like a slice of a helix, that was the idea. The claim is that different angles around the x axis, that is in the y-z plane, would have different phase associated with them because of the forward displacement if the dish surfaced varying with angle.
This makes no sense to me at all. Indeed you will have some kind of helix-shaped wavefronts coming of the thing but I do not see what that has to do with polarisation. And has shift modulation of any kind is hardly a panacea - we already have PSK that alters phase to carry information so that avenue is already exploited. One commentator said that the butchered disk was simply an example of spatial diversity, and we already have MIMO and various techniques allowing orthogonality to be exploited.