Horn's piece is called Concert for Anarchy and dates from 1990. I always assumed that it referenced Cage.



I think that the crucial aspect of the Horn is the way in which the piano is operated mechanically over time rather than it being suspended upside down (though that's obviously a very striking element of the whole thing).
Horn is German and has been involved in sculpture, music and film, and their inter-relationship throughout her career so it's likely that she had come across the Heyn or that they channelled inspiration from similar sources.
There's a biography and images of her work at the Tate website.

To a certain extent I share your scepticism, Ian   However, there are models for it, as I hinted at in my post...  Wagner not only composed the music for THE RING,  but also staged it and conceived the scenery required (artists realised his conception, but strictly according to his specifications and approval).  Menotti (although I wouldn't rate him alongside Wagner, perhaps) staged all of his own works (he was an accomplished stage-director in his own right, and staged many mainstream pieces - Verdi & Puccini - for La Scala and elsewhere).