I'm not sure about the efficacy of such a shelf as you describe- such shelves are normally associated with storage heaters and are necessary where such heaters are located on solid walls as opposed to below windows. The reason being that the movement of air over the radiator causes "pattern staining" of the wall behind, as particulates carried in the air curent cling to the cooler wall surface. Many myths abound about siting of radiators but most heating engineers agree that the best location for a radiator is beneath a window, the cool glass surface encouraging the flow of the warm air up wards and into the room, circulating the heat.