"Before their Baptism, certain Japanese were greatly troubled by a hateful scruple: that God did not appear merciful, because He had never made Himself known to the Japanese people before, especially that those who had not worshipped God were doomed to everlasting Hell. They grieve over the fate of their departed children, parents, and relatives; so they ask if there is any way to free them by prayer from the eternal misery. And I am obligated to answer: there is absolutely none."
Saint Francis Xavier
I think that Saint Francis Xavier erred. Here is a completely orthodox path of salvation for those Japanese peoples who lived prior to the coming of Saint Francis Xavier:
1) As "anyone whatsoever" (Fourth Lateran Council, Canon 1) can administer a valid sacramental Baptism, and as for an infant, a valid baptism is always a fruitful one, it is possible that some of these "Japanese ancestors" were sacramentally baptized in their infancies, perhaps even by angels who, Saint Thomas Aquinas teaches, can validly administer a Sacrament.
2) As the One and Triune God can raise individuals back to life, so, too, He can grant "salutary repentance" at death's door, so it is possible that these Japanese ancestors, especially those in Category #1, received forgiveness just prior to their particular judgments.
Positions #1 and #2, by the way, are explicitly espoused by the Saint Benedict Center in New Hampshire, if only as possibilities but not as certainties.