I wonder actually, if by using
Java MOBI metadata editor to forge the ASIN, will the Kindle Fire think it is a regular Amazon book and show the covers?
For background: Due to the disaster that is/was Cloud Collections, I came up with the idea for somebody to add Amazon ASIN data to personal docs so KPW2 would stop bugging about it being a sideloaded book. According to theory, it should just behave however Amazon treats any book that comes from them, instead of as a PDOC, which sounds like it might be the problem here.
I have no idea if this would work (does calibre even send the thumbnails to Kindle Fires? I know it does to Kindle e-inks,) but I suppose it's worth a try.