Data point: I booked a TPAC on UA with CX segment to MY (PE on UA, Y on CX). I have OW Ruby (AA Gold) status. I was able to add my OW number on the CX website, which only shows the CX segments. I was not able to select seats for either segment on the website. On the CX app, I was able to select a regular or preferred seat for the first flight, but I was still receiving an error for the second flight. I contacted CX via WhatsApp, and was finally able to select a seat on the second segment (the process was tedious, waiting 10-15 minutes between each reply). Unfortunately, my second flight from PEN to HKG is on the 32Q. The only silver lining is that exit row 35 is only listed as a preferred seat, so I was able to select it. Exit row 55 is an extra-legroom, so I was not able to select it.

I primarily fly *A, so I am not terribly well-versed in the OW rules. I know that BA only allows free seat selection for OW Ruby starting at seven days before the flight. I initially erroneously assumed that was a OW rule, but I have learned that this is not the case with CX. Had I waited until the week of, I probably would have missed out on getting an exit row aisle seat altogether.