Here are my thoughts, though far from complete. When you say "off the coals", I'm thinking you are talking about the planks being at grill level? The bisquette burner gets around 550°F, a little lower than the grill surface (depending on how much fuel and the distance from the coals). Though the wood is smoldering at approximately the same temperatures; you are cooking/smoking at a much higher temperature, when using a grill versus a Bradley, your food will be cooked/smoked in a much shorter time. The food will be exposed to less smoke. I would guess if there were a Cedar bisquette, you still would have to be careful on how you apply the smoke. With the Bradley, you would only apply a small amount of smoke to your food. If used improperly, the food will be over smoked to the point it could be inedible. That would mean less consumption of cedar bisquette (lower sales), and a lot more over smoking the meat (more complaints). Neither of which Bradley would want from its product.
Though it does work very well with plank grilling, because of the oils, resins, and other compounds in cedar, I'm not sure if I would through a chunk of it on coals, when I am cooking a brisket or shoulder; for a long cook/smoke. I've never plank grilled with pine boards. I've used cedar, oak, apple, and alder.