Actually, such rules appear in MiniHB and Heroes of Battle. If I remember clearly (HoB) there are two types of volleys, regular ones and concentrated fire. The basic system is to have the commander of one unit do a special attack using it's int bonus against the AC of the target unit, and the everyone in the target square(s) gets a reflex save (DC 15 + arrow enhancement). In the case of regular fire, the target zone is a rectangle of the same size that the firing unit, and each target creature inside the zone saves or takes one arrow. In the case of concentrated fire, each archer fires as many arrows as it can in one round, and the creature in the target square takes 1/4 of the arrows fired and saves for half damage.