BRU-NRT/HND is full of connecting traffic.ANA will only fly to BRU from Japanese airports which warant a flight since I don't think there is very much connecting traffic between BRU and the Japanese market. I have always been told that NRT was a good market for SN, so this could be an option. But ANA will have to rely on the O&D market and even then, most pax flying to BRU will probably be Japanese tourists, so the yields won't be top of the market I guess.
BRU passengers can connect to all major cities in Japan through HND and vice-versa.
Also, Tokyo area, a 20 million catchement area will have access to SN's European feeding.
NH is more a matter of when, not if.
I expect NH to choose for HND service, but with B763ER rather than 787, initially, followed later on by B777 service if they succeed to growing the route.
Even the initial batch of 787's should have the range to handle BRU.
The Belgian tourism organisms shall also try to convince Japanese to come back to Belgium, I remember flying in the '90's to Tokyo on fully packed B743 and A343. The Japanese tourists and tour-operators are high-yield, unlike other tourists.