You’ll be ticketed in the economy cabin between HKG/TYO/HKG
If you really wanted to sit in premium economy the whole way, HND (CX548/549) is guaranteed a 77W on the timetable which has premium economy. NRT has a few 359 flights which also have premium economy.
You’ll just need to search through multi-city to find the premium economy flights.
Side note, the multi-city search tool is pure rubbish and constantly crashes!