Not however useful for visitors
HKCAD only allows members of the club to fly, and only HK residents if I am not wrong. If you are a resident, you can take a 'trial flight', which makes you a member for a few hours. As visitor, I don't know. I guess they would be breaking or bending the rules if they allow trial flights for visitors.
There is also the fact that GA is only allowed on Saturdays and Sundays.
Helicopters fly all week long. For airplanes this could indeed be true, but probably because they are not allowed to use the Shek Kong Airfield during the week. Shek Kong Airfield belongs to the Chinese PLA (People's Liberation Army), and that's where the club's fixed wing aircraft operate out of. Chek Lap Kok (HK International Airport) is not open for GA under normal circumstances.