I agree. Also, since cast iron is a reasonably good conductor of heat, instead of the lid being heated by the steam, it could direct transfer the heat from the lower pot, get hotter, and defeat the regulated temperature intent of a double boiler
What I do do is; if I'm slow cooking something in this; I'll wrap some food in aluminum foil (corn, king oyster mushrooms etc..) and put it on TOP of the lid, cooks well. My favorites are vege packs though. Any vegetable you want + a tablespoon of butter + a few shakes of Old bay, seal the aluminum foil pouch, throw it on top of this when you are cooking, in about 15~20 minutes, BAM! vege heaven.