I guess the title says it all. Short of creating a page with a custom template that links all the posts with a given tag (this is NOT a scalable solution, particularly if you want to let a customer do this), how would you add a menu item that takes you to a tag archive?
What is the best way to create a menu item that links to all posts with a certain tag?
When setting up custom menu click
Screen Options(top right, nearHelp) and checkPost Tagsthere. You will get metabox that will allow to add links to tag archives as menu item.