Injecting a javascript banner code right before & after wordpress navigation men

It’s difficult to inject content from a plugin precisely because themes use different ways to display content, not just IDs and classes. For any theme that uses wp_nav_menu() to display navigation, use the ‘wp_nav_menu’ filter to place your own empty divs before and after the nav HTML. You can then use javascript to fill the divs with whatever. I would hope nearly all themes use this function, but there are no doubt a few mavericks that do not. Unless you want to develop custom code for each of these, they are pretty much a lost cause.