I've always been under the impression (and Prince's actions seem to match this) that the Warner contract only covers material released.

In other words, Prince signs a 3 album deal. To meet that contract, Prince has to submit 3 albums, and those albums have to be accepted by Warner. They have the right to refuse, but in doing so, they cannot claim ownership.

If they accept the album, they are given (and hold) the masters for manufacturing and contractual purposes. they are holders, not owners, for 25 years.

In the case of the 3 LP Crystal Ball, they rejected it, so therefore, any track that did not get a subsequent release while under contract would not be owned by Warners in any way.