There is no way of having dynamically changing messages at the loginwindow without constantly killing it. Besides, you might not want people logging in while your first-boot scripts are running.

The solution to both of these is to use iHook. It does take a little getting used to how it works, as you need to use a launchagent in the loginwindow context to run it, and then need to have your master script be called by it (unless you get really creative with things) so that it can get your stdout to display/change things. But there are a lot of people using it, so it is a workable tool.

Edit: Ok, this “spam post” thing has really got to stop. A couple more times and I will never post here again.

Sorry “punk” but you are going to have to google iHook yourself.