Yes, it is same plane and single boarding pass, deplaning and security check required. At one point transit passengers got a SQ sticker put on their clothing indicating they are transit passengers (not sure if they still do this though).

If you have lounge access, usually there is enough time to visit the SQ lounge, and staff will make an announcement (or move through the lounge alerting passengers) when the plane starts boarding again. Typically, SQ seem to have gates reasonably close to the lounge at HKG.