Having done a Mobipocket Quicklook plugin -- see
https://www.mobileread.com/forums/showthread.php?t=25603
-- I thought I have a go at a BBeB/LRF/Sony Reader plug-in for the Mac OS X 10.5 (Leopard) QuickLook system. This is only for unprotected LRF files.
I found it easy to extract a small GIF from the file.
[1.1 update]
With a lot of assumptions, I may have found a way to access a higher resolution cover if it's present in just the right way.
Just download the attached file, decompress it and put the result in your /Library/QuickLook folder. Your LRF eBooks should now show their pretty covers in icon view, and in coverflow view.
Only for Mac OS X 10.5, PPC or Intel.
[edit]
This is version 1.1. I have a very limited number of LRF files to test with, so do let me know of any problems.
Paul