The Hang Seng is less weighted towards mainland sticks than the HHI even
Are you sure your general hypothesis is correct? I can't seem to find the link but I read something just today about how the Hong Kong market is where a lot of the larger Chinese companies are, with the mainland exchanges more dominated by smaller firms.
I am not sure it is safe to make macro bets on an intraday scale, so I wouldn't worry too much about missing out, could just as easily have cut you up the other way.