The only way I can think of doing something like this would be to have a Windows service use the System.Filesystemwatcher class to watch the IIS config file, and when it's changed maybe parse through it or something to look for new sites.
IIS Modules and stuff only work when you get an HTTP request, right? I'm kind of new to IIS extensions, so that may be wrong ;)
http://msdn.microsoft.com/en-us/library/system.io.filesystemwatcher.aspx