That's exactly what I'm looking to do. In fact there was one of the Canon 70-200 L f/2.8 IS USM's, with the "doubler" for sale on Craigslist. I don't know if it's still for sale or not. It was priced a bit higher than I thought it was worth, but the person answered my emails, and he had used it for both outdoor sports, and some wildlife.
I was also looking at the 28-300 Canon, but wow they are proud of them!