Travelon Gamelon (1977) by Richard Lerman is using piezo-ceramic disks as contact microphones. Many of his works use transducers and his website is a very valuable resource on using these, including a guide for making these microphones.
Andrea Neumann performing on her Inside Piano:
Wave Train (1966) by David Behrman performed by Yoni Niv (Musica Nova) and Ronald Boersen (special guest). See also: Sound in a Jar by Ronald Boersen.