Hi Andy, I've been using a garment like this for a while. I just ordered an eVent Packa, though I haven't really been able to use it much yet. IMO this type of design is the most functional rainwear garment available for backpackers.
Maybe the easiest way to make one is to use the Parcho kit from Quest. This is zippered, rather than using snaps, and includes everything needed for construction. I like a two way zipper, although I can see how snaps/velcro would lend nice functionality. If you build one, here are a few tips:
Make sure it's nice and long. I like the bottom hem to be just above my knees; it eliminates the need for rain pants in a majority of situations, especially if worn with tall gaiters.
Make sure it has plenty of girth. Remember this is going to fit over your pack harness, so without a pack on, it's pretty big and shapeless. You want plenty of space across the chest and under the arms for air movement, and also to allow easy ingress/egress while wearing your pack. Space in the chest also allows the carry of a camera case on your chest or waist, another favorite feature of this design.
Use some cheap fabric and play around with the hood pattern and dimensions. For me, this is the trickiest part of making a jacket, and it's make or break; if the hood doesn't fit well, it really compromises a jacket's functionality IMO. Just make a few mock-ups, paying attention to the height of the back (you don't want it pulling/binding around to back neck seam) and the coverage over your forehead (you want a brim long enough to keep rain off your face). Try using the "nevasnag" from thru-hiker to line your hood brim; I've never used it but have heard it's the stuff for stiffening brims.
Good luck! People are really struck by these when the see it in use; you'll get lots of questions on the trail.