Beth writes, "I've started on the back, seems to be going smoothly so far - I just find Kate's design with that back pleat to be ingenious. However, I think I need to make mine a little longer than the pattern calls for - and feel rather intimidated to figure out how without goofing up that pretty pleat as well as matching up the front pieces!"
I think that lengthening the body would involve a bit more than just working it longer than instructed, because of the back pleat. I've never modified such a tailored garment before, so if anyone has some advice, please shout out!
My guess would be that in order to keep the armhole shaping intact, the length needs to be added to the body pieces below the point where the armhole shaping begins. On the front pieces, this isn't much of a problem, as you can either add some rows in at the "continue even in patt until piece measures ___" part, or add two rows in between the decreases (as I did for the sleeves), to keep the line of the side seam straight.
For the back, you might be able to add a bit of length by widening the yoke piece, and then repositioning the armhole shaping on the back, but you either have to know ahead of time that you will adjust the length, or be prepared to reknit the yoke. This would be considerably fiddly, as well as setting the top of the pleat further down on your back, rather awkwardly. The diamond section on the pleat is the same on all sizes (112 rows), and so obviously the least-complicated way is how Kate set the stitch count for the differing sizes, by adding stitches to the plain sections on either side of the pleat, and then at the "cont without further side decs" section (just before the armhole shaping begins), do decrease until the correct number of stitches for your size is reached. The pattern doesn't give you much space to perform this trick, but I think it could be done, as long as you don't want a lot of extra length! It might be simpler to put the side decrease rows closer together, spacing them out evenly through the stockinette section.