I'm skeptical about whether this will work. CX does overbook often, but they're most likely to op-up elites instead of leaving someone behind. I can think of only one flight I've been on in the past few years when they were asking for volunteers. I assume only your TPE-HKG-SIN is on CX -- the rest is on MH? Frequencies are so high on those routes that you're more likely to be rebooked on the next flight an hour or two later.