OLCI won't work since you can't print boarding pass ex-SIN. Not sure you will be able to pick up your SIN-NRT boarding pass since you can't check in unless it is within 24 hrs. With the BKK-NRT left at 6:50am, I am pretty sure their checkin desk will be close by 6:45am. Last time I did a similar trip flying TG on award, I end up paying 13 SGD to put my bags at left luggage and head to town for a walk and end up having a late dinner. Took a cap back to airport around 3am and wait for UA counter to open.