In the case of ICN-NRT, it cannot even be booked online in F when I checked, due to a bug in the system that thinks F isn't available on that route. And AFAIK, even if it did work, there's no way to just reserve a flight online. You either need to book it then or nothing at all.

Calling always works for me. Anytime I think there's even a small possibility I may want a flight, I'll reserve it and then cancel at some point later if I find out I don't need it. There's some booking classes that must be ticketed a week in advance, but most require only a day in advance or even the same day of the flight, being D+, because the ticketing deadline is waived for D+. There's a few countries, namely the Philippines, where things are a bit different and you always need to pay for the ticket at least one day in advance if departing from there, and even then they make a lot of noises if you don't pay at least several days ahead of time. In my opinion, the waiving of the ticketing deadline is one of the great perks of D+.