I snagged F seats on JFK/HKG flights but AA could not assign seats but only request them. I did this with the Expat desk and they said that CX would have the assignment listed in 24 hours. Looked up my reservation on AA the next day and my seats were there. If you go to the CX website with the CX PNR you can view your seats but not change them or update your information. For that it requires the 10 digit ticket number (not the AA 001 number the CX 160 number) to unlock your record. That I don't know how to obtain and since my seats were what I wanted did not investigate.
Flying CX F using AA miles this week. For me, ExecPlat desk could only 'request' seats - they told me 1A and 1K were taken and requested 2K for me. However I managed to login to my booking on the CX website and sure enough 1K was available so I chose that.
No problems in changing seats online without the ticket number, however I noticed that some of the other functionality (special meals etc) required the ticker # (which is actually given on the AA E-Ticket confirmation email).