I love the idea of being 'punished' for having writer's block. After all, this is a condition that simply does not exist. I mean, be honest: if you really have so-called writer's block, all you need to do is write about that. Hardly a difficult task.
Writing against the clock for fear of losing anything you have managed to write would, I should think, be another effective antidote: nothing like a mild panic to get the mind into gear!
If all else fails, you could always stand up, take a deep breath, and talk about the first random topic you come to in a dictionary or encyclopaedia and see where that gets you. (Locked up would be a distinct possibility if you tried that in public, so don't!)
I've just written this using the online version of Write Or Die at http://writeordie.drwicked.com. There's a desktop version too, for $10. So what does the application do? Apparently, the desktop version won't let you save anything until you've reached your target word count. There's even a setting for making it delete what you have already written if you're not quick enough off the mark.
A great idea; a great, darkly humorous, use of technology. There are myriad possible applications in a school setting. Discipline problems? What discipline problems? The kids are too busy trying to beat the clock.
Here's a badge I 'earned' while trying out the online version of the program for this article:
And here is a video about the product: