Hey there, long time no see!
For the record, the past practice of providing overseas candidates with an accelerated interview has gone. Locals and international candidates are now screened together in Stage 2, so in any case they will be under a common selection schedule.
As for your question, unfortunately I can't give you anything concrete. All I can say is that I have some friends who are locals and were applying for the first time, and they were called within 1-2 months of their application. I can't tell you exactly how long it takes to get from Stage 1A to Stage 3 either, but a ballpark figure would be around 2 months. The 2 months starts from the day you do the Initial Test, and ends the day you do your Management Interview. Hope this helps.
Perhaps you can send them an enquiry e-mail stating when you will be in HKG, and ask whether or not they can accomodate you. Only the recruitment personnel can answer your questions. Good luck!