the Demontable is interesting but it's heavy as the tube "plugs" are pretty thick. Bill Stevenson built at least 2 bikes like that as well
the Ritchey Breakaway couplers are available as a kit. Not sure if they could be back fit into an existing bike - the seat lug would be a problem.
But visually you'd never notice. S&S couplers are visually obvious.
/markp