SQ seem to make a habit of blocking seats in vast chunks of their Y cabin, only to release them nearer to the flight date, so its worth checking back often for availability.

In particular, at midnight (Singapore time) on the day 72 Hours before flight departure they seem to release seats. If you have KVS or similar you can use this to check how many seats the airline has blocked on the flight. (send me a pm with your date and flight number if you want this..)