I just completed a NRT-SIN RT this week and had an N fare that didn't allow seat selection ahead of time on the return. I was able to select a seat and checkin about 30 hours ahead of time. It may have been avail earlier but I didn't have time to check.

The JAL website is pretty horrible and the checkin/seat selection required multiple pages of effort to confirm my details, and then select from a very basic seat map. It appeared to fail but apparently worked as I had the seat when I checked in the next morning. On the positive side, the gate agent was very helpful in finding me another seat without someone sitting next to me.