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

these look good, but my frame is all chrome, so a black patch might be more visible than a silver ring (if it's thin, like in the first image)

For a chrome frame, I would recommend getting the clamps you mentioned in your OP and silver/white/clear cable outer.

http://www.chainreactioncycles.com/Models.aspx?ModelID=23407