Just a word regarding flights to BKK or KUL, it's still a good 6-7 hour flight on to Tokyo from there and intra-Asia J fares tend to be very high.

I'd suggest looking at Shanghai as BA seemed to have bucket loads of availability on that when I looked, and that is about 3 hours onwards.

My own case last year was booking from SFO (As I'm Mexico-based). Found a few days with 6xJ open, but this was for flying in January. Obviously not suggesting SFO as the option, just that wide J avail can occur.