The Led Zeppelin star joined the prog-rock pair for a string of recording sessions shortly after John Bonham's death in 1980, and he insists the time is right to release what they came up with.
He tells Rolling Stone magazine, "They had some interesting stuff... It was good synchronicity.
"Chris had this wonderful name for it - Xyz, because it was ex-Yes and ex-Zeppelin. Then it was clear that the person who was mediating was approaching Robert (Plant) as to whether he would like to come down and have a listen. Of course, he wasn't interested at all.
"But I'll tell you the material was good. I have the multi-tracks. I hope they see the light of day."