About a hundred years ago when I played centre half, we toyed with zonal defence from set pieces. We fell into the same hole that Liverpool did last night. Conceding goals with everybody looking at each other and nobody being too sure whose fault it was.
In the end, we dropped it and went back to a man-marking system, keeping the zone on for front post only. We didn't mark the posts either, the feeling was that you invited scrambles and scrimmaging around the keeper if you played everybody onside all the time.
Despite the failure of us to defend zonally with any success, I always felt the system was sound in theory, just takes a lot of practice I guess. The main difficulty it seemed to me was counteracting the most basic human principle of all when defending, the ball-watch. They do it with success on the continent though so given Rafas proven quality as a coach, he could well make it work here too.