Can confirm Qantas won't let you check in early at HKG despite other QF flights open for checkin. Was there incredibly early due to a 14 hr layover (and bags could not be checked through to final destination).

A 30-min each way trip on the train to the rail checkin had that sorted though and through to the CX first lounge an hour later.

Unfortunately SYD does not have off-airport checkin facilities.