Sorry to dig this up but similar issue here. HKG ICN YYZ, HKG ICN is a codeshare with CX (more specifically it's actually an interline flight with CX).

When I added the Amaedus PNR to my Cathay one world account, it found my booking. However, I wasn't actually able to select seats since apparently my ticket is as "Economy Light" (which according to CX's site, isn't entitled to advance seat selection).

Selecting seats through AC's site (through "Change Seat" under "My Bookings") results in "we are unable to select your seat for this flight".

Does this mean I'm screwed (I booked the ticket as PY lowest ticket, but I presume seat selection/etc. terms are set by the operating carrier?) And for future reference, am I supposed to be selecting seats through CX or AC's site?