Yes, it could be done by a plugin. It probably wouldn't be a very nice plugin, it certainly wouldn't be very efficient to do (mostly because it's not possible to do it *that* efficiently)
Though, I'd argue that finding topics by starting letter is not something most people need to do that often, and when they do, they can switch to sorting by letter and paging through. In 6 1/2 years of running forums, I've only had to worry about sorting by alphabetic the grand total of twice, and one of those was involved in testing the changes I added to Wedge for being able to set what the default ordering of a board is and whether the user actually has the choice (you can set a board to default to, say, first post rather than last post, and you can force it so the user can't change it, but it doesn't add any new choices, only the ones actually present in SMF and Wedge already, whether there is an interface for it or not...)
Posted: December 13th, 2012, 11:19 PM
Oh, there isn't an interface for this just yet, but the ability to set the ordering on a board has been around without an interface for months. Just a quick DB edit away, for now.