You could do it in OS 9 by switching resolution while DVD player was active, but this never worked in OS X.
Maybe this can be done through an alternate DVD player app like VLC?
Considering the abundance of Mac models and Apple display models with widescreen orientation, this would appear to be a no-brainer. There are a lot of standard orientation letterboxed DVD titles around. Shouldn't there be a DVD player app that has such a feature built-in?