To do this, I edited this file:

/blogs/plugins/_quicktags.plugin.php

I added a function called EscapeTags to do the replacements, and a function called b2evoEscapeTags to get the selected text, pass it to the first function, and replace the result in the text. Finally, I added a new button to the end of the toolbar, modelled on the X (Close all tags) button. These last two buttons call special functions, whereas all the others just call a generic function and pass their open and close tags into it.