My experience has been don't expect it, and if you happen to get it you will be very happy. When I was Platinum (5 yrs straight) I got op-uped maybe 40% of the time when flying coach.

I found it doesn't hurt to politely ask at the gate ticket counter if there is any chance of an upgrade. Platinums will be ahead of you and there are usually several of them on those Tokyo flights, so again I wouldn't expect anything.