I took a look at it, and it looks like you have to make a plugin for each app you want to support. Also, it seems that it can only be used by programs that do not use a custom subclass of NSApplication, but I'm not too sure about that.
I think it might work better if one was to create an APE module and put in a custom method for setApplicationIconImage:. However, there are problems in that not all changes in doc icons reflect status that the user should see, such as the bars in Activity Monitor.