OK, got my missing ratings.
The case_sensitive actually didn’t help. The actual files getting scanned are on an ext3 filesystem, though they come originally from a Mac.
I did go through and make sure the case on all directories was the same on both in the firefly library directories, and under the original itunes library directories, and that the case was correct in the itunes xml file.
Once I had everything consistent, I copied the xml file over to the firefly library directory, and rescanned. Ratings and playlists then got populated correctly.
The only things I have out of sync now are songs with non-ASCII characters in the title, but I think that just requires turning on utf-8 support on the NSLU2, right?
Whew! Thanks, guys.