I agree that this would be an awesome Riff. Dennis Hopper's scenery chewing would bring cause a whole lot of Ham Alerts.
However, as Costner movies go, the best for Riffing would be "Robin Hood: Prince of Thieves." All things considered, I think it's a shame that MKB haven't tackled a Costner movie. He has so many movies that are deserving of the treatment. Even "Dances with Wolves" and "The Postman" (Hell, especially "The Postman") deserve the full Rifftrax skewering.