This is clearly the most complicated extension I have ever developed. Google Hangouts uses Google Wave Gadgets from Google Shared Spaces. This extension taps into the wave technology directly and intercepts all the messages and has a basic API that the extension developer can send messages back to into the wave.
This uses techniques that I have implemented in the Facebook Friend Exporter that I explained on my outdated blog http://goo.gl/tZiFh In order for me to modify the internal JavaScript variables from Google+ Hangouts, I had to do unique messaging, that allows me to transfer data from many contexts.
DOM JavaScript Context <-> Content Script Context <-> Extension Context
Note, this is just a technology preview, I bet it is 99% buggy but it works :) Install from the gallery: https://chrome.google.com/webstore/detail/kdeeabahpojjdkhocdedhfcldgegnmcc
