Hi Alistair,
Yes, I know the topic well--lecture on it often. And I also had concluded that the answer must lie somewhere in Busoni. But despite owning a very large amount of his piano works, still have not found what I am looking for. Which does not mean that it there, somewhere, in some piece I don't yet have.
Geoffrey Madge told me that in one of Busoni's Etudes there is probably a Ped. III marking. Must find these some day. Just too busy with 10,000,000 other projects. (Just this evening, in Paris, I gave a
lecture-concert on piano works of Copland, Cage, Feldman, Brown, Scelsi, Messiaen, Boulez, Stockhausen, Ligeti. Yes, I mentioned the Sorabji Ped. III situation.)
People (pianists) who don't know about playing forte while using una corda are total dolts. And Ravel (and so many others) would not disagree with me.
Best,
Jay