At this point I have both control panels prototyped on breadboards and talking to each other quite well. I haven't been able to test them pointing to each other yet, but that will come soon.
The arrow is also done and working. For this I rolled my own servo from a motor and continuous potentiometer that slides around the shaft. This way the motor can go more than 360 degrees and still stop precisely where I want it to. The arrow is 3D printed and connected to the motor via a spring and a couple of beveled gears in an actobotics aluminum channel. The spring is to provide it some flexibility, in case the bike falls over, and the gears are to keep any of that torquing off the motor.