Cathay Pacific and Dragonair are a bit different than most carriers, in that for economy award bookings, you cannot reserve a seat. I don’t think this is fair, but if they want to treat non-revenue economy customers as second class citizens, so be it. I have booked business class awards before and they do allow you to pick your seat upon ticketing for those.
One of the ways to still try to snag a decent seat is to utilize the online check-in feature. You can check-in online up to 48 hours in advance of the flight’s departure time. If you don’t see a decent seat, you can also try and wait a bit and keep checking as it gets closer to departure, just don’t actually check-in. Lots of times people will change their flights or get upgrades so seats generally open up.
How to know what is a good seat:
I use a few seating chart web sites to help me check for the best seats on planes. They’ll not only give you comments about each seat, but provide information such as seat pitch and where the TV entertainment is located.
Seatguru I find the fastest, easiest, and most accurate.
SeatExpert tries to be smart by looking up the plane type by flight number, but the problem is sometimes it’s not completely sure and will ask you to choose the equipment type after the search, so really not that smart.
Seat Maestro is a new site I haven’t used yet. Taking a quick peak it’s a bit clunky and has a lot of ads, but figure I’d mention it as another option.