Don't see why not. I did this on an award seat out of Hong Kong just last week (although it was on Dragonair rather than Cathay) and have also done it ex-Bangkok. Not sure if you can do it at the intown check in. If I was you, I would head directly for the airport. I timed it so I got there shortly before check in was about to close, by which time they had a pretty good idea there were a couple of spare seats and (after a brief phone call to a supervisor) the check in clerk released a seat to me.