Is there a patch or plugin that enables setting rel="nofollow" on menu items?

I'm doing some experimenting with a "silo'd" theme site and would like to be able to assign the rel="nofollow" tag to a menu item.

The final result desired is to have rel="nofollow" assigned by default (but configurable) to all links except those that are in the content area, which I create by hand.

Thanks,

Ron