Aha, good thought. Reenabling KFX, I get the .azw.res file for comics, but that of course means prose books with illustrations I get KFXes.
Unfortunately, it looks like versions predating KFX (v1.17) don't support higher-resolution files at all.
So leaving KFX enabled, I can at least get full-resolution comics downloaded; but I'd have to get KFX decryption working to get full resolution prose books.
Hmm. Have some thinking to do before buying more light novels.

Thanks for your help!