The Beatles were heavily influenced by the American R&B sound and they had a bit of a reputation for being jokesters, so I still think one of them was behind this. Terry, you rightly point out this this would have been an expensive and dangerous prank, but to me this points to them even more, because they could have afforded to set it up, and they personally may have had the least to lose if the disk wound up being bootlegged, or got radio airplay.
In the go-go 60's the Beatles must have had trusted friends who travelled between England and the U.S. and would agree to such a stunt, perhaps as an untraceable gift or tribute to the people who contributed to their sound?
An inside job, with the help of an employee (or owner!) who had a key, but never said a word, seems unlikely. A private investigator who had break in skills but could be relied upon could be hired to plant the disk, keep an eye on things, and then retrieve it. Think of the Great Train Robbery, D.B. Cooper, Watergate... Seemingly impossible things happen that none of us could ever anticipate or explain...There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy...
Terry, let me turn this around and ask, how do you think it happened? Do you have any gut feeling, or theory?
Oh, and one more question, do you think the vocals on your mystery recording are the same takes as those on the commercial release?
(By the way, I have a small and slightly funny Ringo session story of my own, but it doesn't belong here, so I'll save it for one of Terry's other threads!)