Just an update, I finally got it to play for the first time. Apparently not all USB drives are created equal. I was testing using a 2GB flash drive by a noname manufacturer (branded with another company's logo and given away at trade shows) and that was at least part of my problem.
Tried out an old 1GB Lexar I had laying around and it worked perfectly (FAT32, 16KB clusters). Doing some additional testing with filenames and ID3 tags, but it's worth noting that at least some USB drives simply will not work.