Ah, the smell of a new project. Everything fits together perfectly on paper, and no real-world obstacles have been hit yet.
As far as communication with Flight Simulator X, it has an API for 3rd-party integration (SimConnect), but I haven't been able to find any documentation for it. I suspect it's a C++ SDK, though, which means I'd be writing a PC-side application to talk to it. Instead, I'll be using somebody else's application, called fslink, which is specifically designed for talking to embedded processors over USB/serial for custom instruments. They even have a ready-made sketch for a radio panel available.
I'm planning to make a temporary enclosure out of foamboard to hold things together. I've built a couple of planes out of foamboard from other peoples' plans, but this is the first time I've tried to design something from scratch. It's kind of fun to draw it, cut it out, and see how it actually goes together. 90 degree angles are easy with foamboard, so of course I'm building it as a triangular prism just to be difficult. The piece I took a picture of above was a test article; the real one has to wait until I get the LCDs and get dimensions for everything. My buddy said he could handle making a heavier-duty enclosure for it, so I'll leave that up to him.
To keep the pin count down, each 7-position selectors will be wired through a resistor network to an analog input. I don't have the selector switches yet, but this would be a good candidate to start breadboarding and testing resistor values. I should also be able to wire up a pushbutton and make sure I have communications all the way through fslink to FSX.