I am so excited. It appears that I have succeeded! Now when you import substrate maps, they will refresh the pre-existing ColoredCubesVolumes objects on screen. There will be a flicker as everything refreshes (can't be helped) but in short you can reimport over and over and over again just by clicking 'Convert' and immediately see your results. You don't have to do some drag-and-drop dance any longer. Couple this with a lot more robustness to the little editor and how it searches for/replaces preexisting materials, and this Substrate Converter is looking shiny and nice.