As far as I can see, these null-coalescing operators would break that model.  The PEP doesn't seem to provide for a "real" magic method allowing users to override the actual behavior of the method.  (You can only override __has_value__ to hook into it, but not define by fiat what A ?? B does, as you can with other operators.)  And I think the reason for this is that the operator itself is too specific, much more specific in semantics than other operators.  (I had similar doubts about adding the matrix-multiplication operator @.)