I had a similar issue last year from EWR-SIN. Lots of dates on CO for the EWR-NRT leg, nothing available from NRT-SIN in Business. (over a month window from Jan – April) However they did let me book the BF to NRT and Y to SIN. (of course at full mileage rate) This was for both the outbound and return. Was told by CO that I would be added to the upgrade list, and if seats were available it would be handled at the gate. Flight was full so I sat in coach. I still thin it was worth it for the main leg to be in BF. Although the return where you sit in coach on the first leg much better than flying 7 hours in coach after 14 in Business.

I would say the gate agents were not that helpful in Japan, put the people in the NW club very helpful.