As two OW airlines (although they are really at each other's throats over bilateral flight quotas and Jetstar HK), you should be able to interline?
Even if QF MEL check-in can't give you CX boarding pass, if you OLCI on cathaypacific.com on the outbound and qantas.com on the inbound they should be able to interline your luggate?
CX HKG was able to give me my IB MXP-MAD BP and check through. But that was on one PNR.