Could you like cut away the corners of the old GBC cartridges to fit into the DS? Like on the back of a GBA cartridge it has to ingroves running up the sides at the back. GBC games dont. so couldnt you just make them?
Although the cartridges would fit if you did that I do not think the DS has the hardware required to run GBC games, hence why they made it so you could not insert them in the first place.