@kovidgoyal: So far, it doesn't seem to help either on the few KF8 files that I tried, but then it's not terribly reliable in the first place... I'll check again after a restart, in case there's some weird caching involved. (Go To > Cover seems to still point at the right place, at least).
I did get to check the KF8 fonts stuff on the actual device, though, and, yep, it works

.
EDIT: Well, after a reboot, it seems to do something: it *deletes* the thumbnail that Calibre built. -_-" (On the plus side, there's no broken .part file, but you still end up with the default crappy thumbnail). So, yeah, still no idea how it's really supposed to work, but that was definitely a bad idea ;D