Sometimes Economy Save puts you in V class then you can do advance seat selection

If you have oneWorld elite status then you can also call to get it pre-allocated

And I can tell you, not ALL seats will be open to you at T-48 if you don't have any status with CX / oneWorld... But usually sharp at T-48, usually there will be some aisle and window seats left, particularly aisle seats since there's quite a lot of it there (4 columns of aisle seats v. 3 columns of middle seat v. 2 columns of window seat on 77W) but don't expect them to be located in the forward section of Y...