If you're saying that Pope Pius X was in error,
Any one who says that the baptism of desire is visible for us and that these so called visible cases are an exception to the traditional understanding of extra ecclesiam nulla salus is in error. It is irrational.This would be an error even if it was said by Pope Francis or Archbishop Lefebvre.
I am not sure what other Church authority you have left to appeal to! Is martyrdom in the Name of Jesus Christ ever "visible"?
There is martrydon in the name of Jesus. There are many marytrs in Heaven. However individual cases would be known to God and those who are in Heaven. They are not physically visible to us.
If you say that a martyr ( who dies with the baptism of water or without it) is an exception to extra ecclesiam nulla salus, then you are saying that these cases are visible in the flesh on earth, to be exceptions.This is irrational.