That's strange as I have an ex EU to BKK (booked last week on ba.com) via HKG and after entering the details on the CX website, I managed to reserve two seats in J.
I did notice however, that after reserving the seats, I got a message suggesting that I could not complete the request and I initially thought that my reservation had failed, however, after checking MMB on the BA website, both CX sectors are now showing the seats I selected as reserved.
Edit to add...
I have just checked my booking and the facility to reserve seats is still available.
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