Not sure of the chance of being bumped to SQ, but here's my experience flying NRT to DPS late last summer in Y. On the return I had an opup into J on both the DPS-HKG and HKG-NRT legs. For the leg to HKG J was full and I would guess some Y passengers were bumped. Most of the passengers were European's on holiday, and didn't seem to have status with CX. I was an SG with QF at the time.