It would sure be slick if the ABV were at the top of a snorkel. It would make a FFM much easier to use on long surface swims. Cressi had a pretty crude one for their FFM in the 1960 made from soldered Copper plumbing parts and Chrome plated. The "valve" was little more than a cap held on the top of the tube with rubber that look suspiciously like an innertube.