You can do it, but you'll need those domestic flights. I went from Sapphire to Diamond while I was in Japan the past 3 weeks. I started at 59K fly-on points before I left and was 103K before I boarded the flight back home yesterday.
If you haven't already, make sure you designate the double fly-on points month for the time you're there. You can rack up points easier that way. Granted, I still flew a crap ton (18 domestic flights total) to get those points.
Also, to JALPak's earlier point, you're gonna have 0 luck getting a ticket during the peak travel weeks (Shogatsu, Golden Week, and O-Bon), and even the week/before after can be bad for finding domestic flights.
Also, it takes 2 days for the points to show in your account, but it takes a 3rd day to have your status update.
Good luck!