[Dead Eddie]

Here's a really quick solution. It adds the functionality globally, so it'll work for your subforum, too (so, just add the display to the forums you want it on).

This modifies forumdisplay.php to only show the threads that start with a certain letter if the url parameter exists. Otherwise, it'll be ignored and you'll see all threads.

The links you'll need to add are in the form of /forumdisplay.php/f=2&letter=a

Let me know if I came close.