How to use yum to reinstall all dependencies of a given package?

Yeh, there's no easy way to do this ... however you can do a few things:

rpm -Va

...this will do a check on all the packages you have installed, and you can then reinstall anything that doesn't verify. Also:

yum reinstall $(repoquery --requires --recursive --resolve <pkg>)

...this will do what you asked for, although it's a big hammer.