In my experience of Cathay you can't. You can call and request a seat, it works the same if you book online -- you request a seat (although available seats are shown only if they're not requested). If you're a OW
/AM member or booked online, you can do online checkin up to 48 hours prior to departure.
Couple of notes though:
1) you can only call the airline direct AFTER the ticket's been issued -- prior to that, you need to get your travel agent to request a seat for you.
2) even if you've done online checkin, your seat is not guaranteed, and you still need to get your BP
at the airport.