I have had a lot of experience in SQ, albeit a long time back but they never put a passenger in a middle seat unless(1) the passenger requests it or the flight is normally full. Yes it is playing the game and me as a passenger have played that game and won and lost. No passenger had a right to block a seat unless they paid for it. I mean to even think about is unreasonable. The best option is to ask at check in and most of the times, if it is a *G they do accommodate. As another have said, is there a passenger there or because it is so empty, did SQ block that seat right now for the *G member?
SQ certainly and most airlines do their finals the previous day and I would like the OP if he can let us know exactly what happened when he flies. Was the flight that empty? By the way looking at a seat map you could never tell the booked load of a flight. So the flight may not be that empty at all.