This sounds like an awesome idea! If we're ever able to have roosters, I'd be very tempted to make this my first project....Right, male chicks would have a white headspot and barring on their feathers (evident at just a few days of age, if the headspot is not clear). It would also be easy to make olive egger sexlinks from a black Marans roo but putting him over Cream Legbars. You could keep them in a single pen, the dark eggs make "dark egg BSL" chicks and the blue eggs make "olive egg BSL" chicks.