I was going through a music rip ROM for Gunbuster when I stumbled across a couple of interesting things. First were some tracks that I did not remember from the game - the sound quality was noticeably different (better) than the in-game tracks. But then I came across one track in particular which grabbed my attention. It was a version of the title song from the anime movie 'Urusei Yatsura - Remember My Love'
The original tune can be found at
(movie ending spoilers before that).
The PC Engine version can be downloaded
here.
I'm assuming that this was idly composed by the programmers and hidden in the data somewhere - I'm not sure how these music rips are created, but I guess they must all be from the Gunbuster code and not somehow stitched in from different sources. I've not come across any other hidden music of note in other games but I'll keep looking.