I've considered using it to organize my speeches for Toastmasters, but I've never actually bit the bullet on that one. As you say, Dramatica is fundamentally about the construction of arguments, so I don't see why this wouldn't work. The Main Character would be you, the speaker, I assume, and the Influence Character would be dissenters in your audience.
On occasion, I've used it to organize blog posts, but not to the full extent. For example, I would create four paragraphs and organize them around Past, Present, Progress, and Future, or by Understanding, Doing, Obtaining, and Learning.