The solution is to use a properly insulated bag, designed for long term cold storage, and not more dry ice. 2 lbs is enough to last 24 hours with the correct bag (however such bags are not cheap as they use space-age insulation!).

All airports restrict dry ice in carry-on, and while HKG is not the TSA, the limit is very strictly enforced as dry ice is a potent weapon. Just be glad that it is allowed at all. If you are traveling in F on CX or BA, most crews will put your stuff into their on-board refrigerator if you ask nicely (they are tight on space but can manage to shuffle things around for a reasonable amount of space).