Most elegant solution to add rear brake cabling to a track frame?

stick these under the top tube, don't bother with having no outer cable in the middle because if it means that you got to have huge silver clamps around the top tube, it would be pretty conspicuous and ugly.

They are made by BBB and are smaller than they look in the picture.

+1

However, a bit fragile. A friend of mine snapped one off by carrying her bike cyclocross-style up the stairs to my flat.