you could try the district council for whatever jurisdiction Causeway Bay falls into (is it Wanchai?).
Good for you anyway. My son was injured by falling masonry in Tai Po last year. The police got involved but I guess it was only because the injury had occurred. The District council also helped out a bit and the building involved had the offending masonry removed afew days later. It was a crumbling old aircon ledge.
It was a bit like locking the stable after the horse has bolted, but clearly the building was still dangerous.
The guy responsible (the person whose flat it was where the stuff fell from) was prosecuted under "endangering passers by" and "failing to properly maintain buildings".
added:
http://www.access.gov.hk/had.htm#wan_chai
Maybe these numbers may help, at least someone there may be able to help you a bit more.