Chord glissando? Was Ravel crazy?
Well chords... they're only thirds, but even so, yes it is quite painful if the keys are not very light.
I wonder, can anyone think of something that hasn't been done on the piano? I know of fists, elbows or even whole arms being used to play huge chunks of keys in some contemporary music. Another weird thing is putting screws in between the piano wires to create ringing overtones. (That was done for the soundtrack of the movie "Grizzly Man".)
What about the piece of cardboard prescribed for the Hawthorne movement of Ives' Concord Sonata ! Use to press about 30 keys at a time. sometimes black, sometimes white. A really weird effect.
How about tuning a piano in quarter-tones? Twenty-four keys to an octave? That's something George Crumb would do!
Earlier this year I attended a demonstration on a microtone piano. It was really weird, the pianist running up and down the (normal) keyboard producing sound waves covering only 2 octaves. This guy (Dutch pianost Maarten van Veen) is really good in this kind of thing. Must take enormous concentration to play this sort of thing.