Tried a workaround and finally got some results.

My friends are in Asia and wanted to use HK as a base to do some regional exploring, and I was able to call and book up KUL-HKG-TPE (stop) TPE-HKG-KTM all on CX, since individual flights KUL-HKG and HKG-KTM were not available whereas the married segments were.

A few days later I was able to call up BAEC and drop the HKG-TPE-HKG sectors, effectively leaving me with the two one-ways I wanted to book originally.

Worth considering if you have several one-ways on CX you want to book by combining them into one trip with a long stopover in HK.