I recently booked some SQ flights in Y through an agent and was surprised to find that I was able to choose seats online after the booking was made. I did use my krisflyer login but that made no difference. I didn't think it would be possible since booking thro an agent but was pleasantly surprised. The flight is a 777-300 which was pretty much full as we are 3 passengers travelling together on 2 seperate tickets(and so wanted to get seats next to each other). We couldn't find any great seats but at least were able to get seats together.