Purchased iTunes media artwork not showing up in finder?

Question

I've scoured google and the threads here on the ASC, but nothing has worked. At first I did not notice the pattern, but now I realize it. None of the album artwork that is embedded into my purchased itunes media displays in finder. All of my artwork displays perfectly in iTunes, but not in finder. I started having this problem today, seemingly randomly. I hadn't downloaded any updates to any applications, nor had I installed any new applications. The only thing I did was import four new cds (ALAC format) and the album artwork shows up fine in finder for those. To be honest the problem could have been going on earlier but I did not notice this until today when I went and looked for my new albums to verify the artwork had been embedded properly.
I tried everything under the sun, including deleting the album artwork cache that Finder supposedly shares with iTunes in the iTunes folder. I downloaded a program called Onyx that someone suggested and rebuilt ALL folder window displays, resetting all folder views. I cleared every cache there was on the computer, and rebuilt all databases, including the spotlight index database. I have literally rebuilt all databases that are rebuildable and cleared every single cache there is (including in my /user/library folder) and the problem still persists. I don't know what it is about the purchased itunes media that doesn't display in finder, but it bothers me. It's embedded.
The weirdest part of it is that in Time Machine, all of the artwork displays in the Finder icons and Finder CoverFlow just fine. But when I restore from Time Machine back into my iTunes media folder, the icons go back to the generic sound file icon in Finder (coverflow, quickview, file icons, everything). The only place the album artwork displays is in iTunes itself. I tried using Doug's applescript for re-embedding artwork, to no avail. I'm at a complete loss.
Even if I manually browse to my time machine backup folder on my external HDD, the icons show up just fine. It's just the purchased files that are actually in my iTunes media folder that don't display.
Some info on my setup:
2013 macbook pro w/ retina 15" 2.7ghz model
OSX ML 10.8.4
iTunes v 11.0.4 (4)
I checked for available updates, there are none, I'm completely up to date. I'm new to mac this month, coming from a long line of Windows computers, so I'm not completely familiar with all the nooks and crannies of OSX but I feel like I exhausted a fair amount of my thinkable options here.

Answer 1

I have given it several days now and still no joy. Details are as follows:
1) I have two collections:
    a) In one collection: All files are Apple Lossless (ALAC) with embedded artwork.
         - There is NOTHING else in this collection - no other media - nada.
          - This collection is about 380GB
     b) The other collection is a mix of compressed m4a's, audiobooks, and iPad apps.
          - The music part of this collection is about 170GB
          - The audibook part of this collection is about 43GB
2) In both cases, the iTunes library and media are in a subdirectory of a Pegasus R4 (4TB RAID 5 array).
3) Issue manifests itself with both collections on on both my systems, each of which has a Pegasus R4 connected via the Thunderbolt port
     - MacBook Pro 15 Retina with OS X Mountain Lion 10.8.4
     - Mac Mini with OS X Mountain Lion 10.8.4 and Server 2.2.1
Again, for my case...artwork shows up perfectly in iTunes and in other tools (Clementine, Logitech Media Server), but not in Finder.
Also - tools that use the Finder's mechanism for accessing album art (Plex, for example) do NOT work properly.
I wonder if it is related to having the media housed on an external (if you can call what thunderbolt does "external") drive.