It will never work, because the receiving party of a work declared to be in the public domain cannot verify that the declaring party had the right to do so. Such declarations could be made erroneously or falsely: just having a registry of such declarations doesn't remove the possibility that there is a true copyright-holder out there who might want to pursue his rights later. All the receiving party could to is claim not to have engaged in willful infringement, because he relied upon the declaration.
Imagine that I registered the song "Lucy In the Sky With Diamonds" and all its recordings into the public domain. As I am not the Beatles, I have no right to do that, and that registration would be false. Using this kind of registry would only confuse the potential market of licensees and clog up the courts with law suits.