I am a frequent traveler on this route. Unfortunately, due to the mistake fares at the beginning of the year it is very difficult to get JCL inventory to DAD when a 320 is being operated, and almost impossible from a non HKG POS. I have found that as a DM you will usually clear into the I bucket (ex HKG ticket) after waitlisting as long as there are many unoccupied seats on the seat map, however, if all seats are taken chances are good that it is not "revenue control" but actual passengers who booked far in advance. If you are willing to pay for J, obviously, it clears immediately.
Never a problem when they put a 321 on the route. By the way, CX seems to prioritize local ex-HKG J traffic over connections on even "non-mistake fare" long haul J tickets. On multiple occasions, my colleagues from N.A. have ended up in Y on a J ticket while I have no problems clearing into the I bucket.