Yeah, Cowell and Crumb are the cats you should look into. Two of Cowell's most well-known works are "The Banshee" and "The Aeolian Harp", both of which involve playing inside the piano almost exclusively. Most of his other works also consist of lots of large clusters, using forearms or blocks of wood. Check out a piece by Cowell called "Tiger" for one example of forearm smashing fun.
Peace,
Bri