Yeah it could be conceivable that OP misses the 24hour cancellation deadline. This is what OP should do
1/ LHR-HKG: award ticket one
Arrive HKG cancel (depend what time OP arrives, there is a HK BAEC number that is answered reasonably quickly during business hours - are there public telephones in HKG? Are local calls free?)
2/ HKG-MEL: book award ticket two.
Arrive MEL plenty of time to cancel
Back in the day with CX 24 hour free reservation hold it was probably enough to "reserve / hold" seat for 24hrs and show that to checkin / border force - dont think they are that sophisticated to check e-ticket?