I would disagree; I don't think that the Holy Office Letter is saying that the "dead are visible."
Since they say there are exceptions to the literal interpretation of Fr.Leonard Feeney, they imply that there are explicit exceptions. If they are explicit then they would have to be objectively visible.Visible? These persons are saved in Heaven with the baptism of desire etc.
Rather, the Holy Office Letter is saying that there are some folks who are outside the visible bounds of the Catholic Church who are in a state of sanctifying grace, which is neither "visible" to them or to us.
In principle yes.
In theory, yes.
In faith, yes.
But if you are implying that these cases are exceptions to the interpretation of Fr.Leonard Feeney NO!
We'll only know who these individuals are in Eternity. You can call it an "invisible set" I suppose.
Now they are hypothetical .This is acceptable.But the moment you imply that they are exceptions to Fr.Leonard Feeney it would be an error. It would be irrational. It would be the dead man walking theory.