http://www.wowhead.com/tooltips I've added these on other sites before, I can't figure out where to add the code in xenforo though. I'd like to have a button in the editor that would add the bb code link, but doing it manually is fine.
______that is essentially what you are trying to do.
However...what I have recently learned is that the best practice is to try to not edit the original templates and use template hooks rather to call in your code. Granted that is just one line but if you have 10 things like that and you have to upgrade you are going to have to revert templates.
I myself had no problem doing the edits for out of date templates but now that I understand how to use template hooks, I am going to redo everything I have customized in the same manner when possible.