There is no guarantee but there is no harm in asking. I think you will find the SIN lounge staff friendly and flexible.
How many people here have used the SIN QF Lounges as arrivals lounges? Last year I was in transit in SIN airport and I was meeting serfty and his wife as they were getting off a flight and they were heading into SIN for the night. As they were not in hurry we went to the QF F lounge and they were both allowed inside and serfty and I had a couple of beers and some food before I went to board my flight and they headed off into town.