Add Link to Nav

I want to add a link to the end of the main (top) navigation regardless of which template is being used or how the menus get configured. It would be nice if I could get it treated the same as all the other nav links (templating treats it like a page). I don’t want to use WP’s database because the link needs to be displayed only sometimes based on data from other sources. I’ve found hooks to add content to lots of parts of the page but so far I’ve only found a function that retrieves this menu, nothing that injects a link.