@kovidgoyal: It's available in the pnp_id on Windows, but I couldn't find an easy way to get to that, so I ended up running the scanner again in my
Kindle Collections plugin tweaks, in order to be able extract it from there with a dumb regex.
That said, I'll look into the whole thumbnails/cover thing once I receive my PW, I have a few things in mind to check... (I'm not sure the bigger thumbnails actually hurt on older devices, for example).