In case anyone is interested, Qatar Airways are doing a flash sale for Brussels to Tokyo in business class for around 625euros per person, return. This is of course via Doha but exceptional value
http://www.headforpoints.com/hfp/qatar-airways/
I don't normally struggle with jet lag too much but Jet lag going east is always worse I find. Been to Hong Kong and took a good 4 nights to get acclimatised and that was after flying First. I'd probably consider a week as long as you appreciate the time difference and think you can work round it