Markkel, in knife edge you are correct that the valve possibly won't seal properly but I think it's really the oil pickup that is sucking air that creates the low pressure. The oil pickup is usually one one side of the sump so knife edge on one side may keep oil pressure but the other knife edge will not. Some exotic sumps have a swinging oil pickup which works like the flop tube in the fuel tank ! Good idea but how much knife edge do you want !?