Virgil Thomson: for Four Saints in Three Acts came up with motifs corresponding to Gertrude Stein's words without writing them down. Eventually when the same motifs had recurred from memory enough times he decided those were the ones worth keeping and they became the opera. He worked the same way with his portraits--musical interpretations of people that resulted from sitting in the same room with them.
Eric Satie: "furniture music" consisting of more or less interchangeable parts that filled space and time but did not dramatically engage the listener
George Antheil: a piece for player piano no longer governed by time as music had always been. it can be compressed and uncompressed more or less instantly like a piece of visual art.
--notes from memory on Daniel Albright's book Untwisting the Serpent. Not quotes and the ideas may be slightly altered from Albright's meaning.