Sure, except I doubt most people want to uglify all their urls with extensions for occasional alternates. Plus, if the url with the extension gets past around instead of the original (as would inevitably be done) you're back to square one.

I had thought about recommending that people just use an alternate link as intended, to point to an alternate format. I think that would work best using existing web standards as intended, but it has the downside of initially serving the original format regardless of the content type.