After much consternation (I was having trouble getting the sketch onto my SparkFun promicro 'cause I'm a dumb) I now have a fully functional USB HID device that my Mac sees as a game controller.
On using Sota's code:
The code Sota wrote for his project seems to be working very well. It's designed for 14 buttons and up to 2 analong sticks. It works MUCH better with both analog sticks installed. I found I was getting a lot of noise when I was just using one, but with two it was quite stable.
Next: a bit of wire hacking. Getting all of the components into the project box comfortably without modding the board is going to requier making some specialty cables and one very short USB cord.