Originally Posted by LHR/MEL/Europe FF

All flights depart from the same departures area at Hong Kong, and you can freely move between all gates 1-72. Smaller aircraft may depart from a remote stand, or the smaller satellite terminal, but you'd take a bus to these shortly before boarding opens.

To progress from arrivals to departures you will need a boarding pass to clear transit security, and once done, you are in the main departures area.

You could use your KA boarding pass (or lounge invite) at that stage to access any of the CX lounges.

On arrival at the lounge your lounge invite (or boarding pass) will be scanned. I'm not entirely sure if this 'confirms' your check-in and your intentions to fly on that flight. In any event, if you decide to skip the last leg to HKT, you should inform the lounge staff so they can off-load you (a simple 'I managed to find an earlier flight on Air Asia' is understandable and will do the trick). If you don't off-load they'll be looking for you.

If your original award ticket is one way only (JFK-HKT) you'll have no problems throwing away the last leg. If the ticket is return, skipping the last leg will cancel the rest of the flights, including your return.