I can tell you all from 'inside sources' that the current MFSL has avoided adding the quad tracks to many titles they paid for exclusive windows on, thus "Quad Blocking" any other label from releasing the quad masters due to the licensing window.
As mentioned earlier, Rhino/WMG can do whatever the like because they are the owners of the titles, but MFSL discs like Nilsson's "Nilsson Schmilsson" and many others were put out by MFSL which prevented other labels like Audio Fidelity from licensing the title.
Not just during the exclusive window. But after the window expired, the cost of putting out a title that was recently released by MFSL was not financially sound as the addition of the quad tracks was not enough to make the general population go out and buy the same title again.
I would have to say that the term "Quad Blocking" would not exist in the world if it were not for Music Direct and their ownership of what used to be a leader in quadraphonics, Mobile Fidelity Sound Labs