I did spend some time working on the car I bought, specifically the computer and sensor system. It is 1990 Infiniti Q45, so its computer system technically qualifies as retro, I guess. The previous owner had put a piece of black tape over the diagnostic information display to hide the "STOP LAMP INOPERATIVE" error. Unfortunately, that was one of the most minor abuses that he put this car through. Anyway, a few minutes of soldering on the stop lamp sensor to fix some damaged solder joints, and I was greeted with "OK"!
It actually took less time to fix the sensor than to open up the dash and get the black tape off!
So that's it. I compiled the new lwip code, made an application, but never tested it, and made my old car computer say "OK".