I own HK set iPhones for many years. But have never sent for repair/replacement (Hope I never will have to)
Apple warranty is international, but the turnaround time for repairs/replacement for HK sets might take longer as they need to ship specfic parts/units from HK.
I rmb a bro mentioning he requested for express replacement unit for his hk set & got it within 3-4 working days (Shipped from HK)
AC+ can be bought locally for HK sets, so in summary I don't see any reason Apple SG won't take in rma services for HK/overseas sets