I'm not sure it's the complete solution, because it doesn't validate in epub, but it does give a thumbnail.
I use sigil to open up the epub.
Make a thumbnail image (I make them 240 high, but I still use the old 800x600 covers) and import them into the image folder.
In the content.opf file, add the code into the <metadata> section:
<meta content="images/image00004.jpg" name="Cover ThumbNail Image" /> (where the image00004.jpg is the thumbnail file name.)
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