Force Theory Productions has a concept video on their website for what they call a human piano. While not necessarily a piano, it does turn people into part of the instrument, using them as electrical conductors.
By running a low voltage current through the human body into a mixer and connecting the electrical current from an iPod's audio output to copper pipes, Michael from Force Theory is able to create sounds with the pipes as he touches them. By squeezing with varying degrees of pressure, different sounds are created. Pretty cool stuff. Check out the video below to see the human piano in the live setting (where his subjects are presumably touching the copper pipes). I kinda feel like I'm watching the demon spawn of Jason Mraz and David Blaine in action.