If it's the category that the items on the page fall into, probably not.  If the inserted keyword leads back to a page outside of the category, it may be a little spammy.  From what I understand, breadcrumb produces codes based on where the viewers was on the site, so if the inserted code looks like all the other dynamically produced html, I don't know how G could distinguish that, other than it doesn't fit the content on the page.

I do think NoFollow is more pertinent for external sites linking to your site, and not so much inner site links used for navigation.  I'd be more concerned with the amount of keywords used in the metatags.