Prelude In C-Sharp Minor Opus 3, No 2- Sergei Rachmaninov
I always Joke with my wife that I want her to have this played at my funeral mass (It sounds very morbid).
The version by Alexis Weissenberg in my opinion is the best (because of the range of emotions expressed). This would definitely fall under "unconventional". My spirit would be chuckling as I watched people glance around at each other with puzzled looks (of course the reaction would be much more subtle than to "another one bites the dust" ha ha)
But I really do think that Rachmaninov's entire 3 movement Piano Concerto #2 in C Minor Opus 18 would be wonderful background music at a wake (the third movement gets a little lively but it still fits very well, in my opinion). My favorite recording is the one from the CD "Essential Rachmaninov"(Concertgebouw Orchestra Amsterdam-Vladimir Ashkenazy on Piano/Bernard Haitink Conductor). The piece has an aura of melancholy which is offset by a simple beauty and love that is hard to describe.