I’m not much of an expert on this, but maybe it would be possible using the [link_after parameter](http://codex.wordpress.org/Function_Reference/wp_nav_menu). If I’m reading that right, it would work like an :after, meaning it is still inside the link. I’m not sure how well it would apply for you, but it might be worth testing.
Yes :after would work, but I opted to go for jquery in the end due to browser support. (Even though I was planning on avoiding this). Simply appending a span tag after the specific class name seems to do the trick.