There is also a Louis Tavern hotel (or at least hotel rooms) located airside at BKK. Checking prices last time it was about the same price as the Novotel per night (USD150 or so?). That would allow you to stay in the F lounge as long as you like, sleep for a few hours in a bed, then head to the F lounge first thing in the morning. No need to worry about immigration, taxis or anything else. *Note there is a small chance that if you are on a through ticket, you could be asked to pay the departure tax of THB700 when you try to re-enter the airport after your stay in BKK. The Louis Tavern option would avoid this.)

All things said, the Fcl is not somewhere I'd really want to go to all the extra hassle to visit... by the time you get there and settle in you could be half way to your hotel in the city. The champagne isn't as good in the lounge as you have on board, and the food is much much better 'on the outside' than in the lounge. The only advantage is a massage, but again those are much better outside the lounge.