See, I've been to other sporting events like the European Cup '08 and the 2010 Olympics where people dress up just as crazily but for something that they are actually passionate about (GO CANADA GO!!! :)).
In HK however, the people who go to the 7s seem to have a) no concept/love of rugby and/or b) no particular fanship to any team. Costumes are thus totally random instead of representing any one country and the point of going seems to be more about getting wasted than cheering on your team.
Don't get me wrong, I too LOVE dressing up but paying big bucks to drink with a bunch of other peeps in costume to (not) watch rugby live just doesn't make any sense to me... ~_^