Tasks are not designed to run asynchronously, so you can make a pop-up box, but the next task wont fire until the previous is exited.
In theory, you could schedule a task that is a custom script that pops up a message AND deploys your software, but then you've lost nearly all of the functionality of the NS.
What you might do instead though is to lock the keyboard and mouse, so that the computer can NOT be used until it's done, and then make your last task an unlock. It's a thought. Several people used to do this during deployment so no one could actually mess-up the deployment of an image.