I use both MMM-PenPlotter and MMM-Wallpaper (the jupadin version) and was able to make the images transparent with no black box. You need to edit each of the svg files in the example_svg folder.
Most of the files were updated by adding
in the section with:
<desc>pwidth:500;pheight:500;</desc> <path stroke="#999999" stroke-width="0.2mm" fill="transparent"
There were a couple where background was already defined in the 3rd row of the files so it had to be updated there. Let me know if you need more specifics.