I haven't found anything definitive, but I think he definitely played part of that section of Close to the Edge on his fretless Guild JS-II, and even used it live on the Close to the Edge tour for that section (with it mounted on a stand) and switching back to his Ric when the vocals come back in. It's interesting to find this stuff being whispered on the internet over the past 10-15 years but not entering the more general knowledge-base, though I guess that's probably how the internet works...
And I apologize for bringing Yes into this thread, though Close to the Edge definitely has some ambient moments!