Proving you can fit the world's population into an area the size of Greater London

I'm writing a blog on festival audience sizes and how much space they take up. `I want to create an example of how the world's 7.5 billion population, when standing side by side and front to back, would all fit into an area no bigger than Greater London. (It used to be the Isle of Wight but at 350 square miles it will now only accommodate three quarters whereas at 550 square miles Greater London is the next best well known geographic area which can do it comfortably, at least of the time being). This is based on a square metre easily accommodating 6 people (3 adults 3 children). What I am looking for is an interactive scale model map the size and shape of Greater London on which people can click and place a pixel sized dot to represent a person. A counter will show how man clicks have been received. The more clicks the map receives the more dots will appear (preferably juxtaposed so that eventually they turn into a growing mass that can be seen when zooming in). I haven't worked out how many clicks would be required before the dots start becoming visible unzoomed but 25 million might do it. I am open to suggestions on the finer point of the development but basically it's a fun way of demonstrating this little known fact. I don't think anyone has attempted this before but I might be wrong. If I'm not it could prove quite a popular site for students in particular. We would have to protect it from spammers and hackers.