travel is an automatic dispensation
By travel I meant while travelling, i.e. in transit. Of course if a Mass is available in the city or area we are visiting, we should make every effort to attend. And if travelling more or less locally by car, there is more flexibility.
But international air travel is pretty much automatically dispensing unless coincidentally Mass is available in an airport chapel before, after or between flights. But even there, once airside in the sterile area between flights, it’s not practical to attend if the Mass is street side or in another terminal.
The OP’s situation includes 20 hours of flying across the international date line. It means probably leaving by midday on a Saturday and arriving late on Sunday in order to be available for the Monday event.
It also sounds like these are parents of an adult child, thus “of a certain age”. Even if Sunday evening a Mass is available in Seoul, it would not be reasonable to expect them to attend. I’ve done this very trip multiple times in my life in my 30s and 40s and even at that age it was brutal.
God commands neither the impossible nor the unreasonable. In the latter category I would include leaving the sterile area or terminal between flights, tagging on an extra day, or venturing out into a strange city in the evening after 24+ hours of travel without sleeping in a proper bed.
Of course going across town to visit grandma is another matter, I consider that a short journey, not “travel”.