Okay, finally finished downloading all DLC from UK XBO marketplace, here are my results...
Missing
The Doors RB3 Free Launch Pack / The Doors Greatest Hits: Special Edition (the version without those 3 tracks)
RB3 Pre-order Pack (Burning Down the House/Blue Monday/My Own Summer)
Rock Band 2 Bonus Tracks
Rock Band Free Pack 01
Alien Ant Farm - Smooth Criminal
blink-182 - The Rock Show
Deep Purple - Child in Time
Disturbed - Inside the Fire
Frankie Goes to Hollywood - Relax (Come Fighting)
Listed but no free download
The Doors - Greatest Hits: Special Edition
Average White Band - Pick Up the Pieces Part of the delisted Get the Funk Out Pack
Deftones - My Own Summer (Shove It) Part of the RB3 Pre-Order Pack
Disturbed - Perfect Insanity
Earth, Wind & Fire - Shining Star Part of the delisted Get the Funk Out Pack
The Jimi Hendrix Experience - Little Wing
New Order - Blue Monday Part of the RB3 Pre-Order Pack
Talking Heads - Burning Down the House Part of the RB3 Pre-Order Pack
Not too many, biggest problems seem to be associated with pre-ordering Rock Band 3 and the Get The Funk Out pack being delisted.
Edit: Forgot the two Disturbed freebies from back in 2008.