Flightdeck visits are only allowed on the ground. Cathay are quite strict on that and we tend to not break the rules on that one.
If you would like to see the flightdeck beforehand, make sure you are one of the first to board when the boarding starts and maybe have a preprepared little note to give to one of the cabin crew who can hand it to the captain explaining who you are and that you would like to see the flightdeck. They will be quite busy before the flight so unfortinately instead of being able to go for a long chat it might be a quick visit of a few minutes. Alternatively you can try to see the flightdeck at the end of the flight, but chances are the crew will be in a huge hurry to leave, so I suggest going before the flight.
Good luck!