Cory, a Brooklyn based artist, one day noticed that only three tracks on Bruce Springsteen's "Born to Run" album are adorned with glockenspiel. He decided to right this wrong by recording additional glockenspiel parts for the six songs on the album that did not contain the instrument. A small run of the "Born to Run Glockenspiel Addendum" was pressed on vinyl and contains only the glockenspiel parts. It is meant to be played concurrently with a copy of the original album. His site contains some background info and before/after mixes (link).
It vaguely reminds me of Redd Blood Cells (when the guy from Redd Kross decided to play bass along w/ the White Stripes, also in the name of art).
Update: In the comments Nicholas asks "Is there a longer history of people recording things meant to be played concordantly with something else?"