I would imagine if there are an empty ferry flights they would be HKG to MNL and return. An aircraft comes off a check and is ferried to HKG. The aircraft which is due a check does the service to HKG, and is then ferried to MNL. The aircraft which came off the check then does the service to SYD. Sound feasible?
Feasible, yes, but then it would entail a change of aircraft for the through pax at HKG, as opposed to what is currently offered as a direct service (i.e. with a transit stop where the onward leg is on the same aircraft.)