Ahh, yup, looks like that’s still there on my version at least (Model 01-Q). I had it apart before (getting the top case back on is a good time, but some nylon guitar pick-like thingies I had from an ifixit toolkit made it much much easier when I finally thought of using them) but hadn’t detached the board/plate assembly from the bottom case. Here’s some clearer pictures:
I wouldn’t guess the pinout has probably changed between production revision but if you don’t have a 01-Q probably worth checking for yourself before you plug anything in.
Connectors are just standard .1" headers, looks like. There’s also separate things for most of the controller pins (female headers), and the controller SPI pins (male pins, on the left when viewed from the top), as well as the ATTiny SPI pins (male pins, on the right when viewed from the top.)
Easiest way to route a cable out of the case is probably just to drill a small hole in the plastic case on the bottom, plus that way you don’t have to mess with your gorgeous topcase. There’s a little bit of room left in the wood case on the inside to make it possible to easily attach things to the expansion port, it looks like.
Anyway, easy SPI attachment is possible, looks like, and I’d guess none of those analog pins are being used either so it looks like I can do what I wanna do pretty easily.