[Michael Hill]

You're correct that it's a safety concern. FIRST needs to be guaranteed that if the refs (or your team) disable your robot, it will be disabled, so if the PWM gets generated by a breakout board (or monkeyed with), they can't be guaranteed the motor controllers won't get a signal. What you can do, however, is make a breakout board that talks to something like a beaglebone or some other platform that handles sensor input and does control loops and interface that with the MXP. I know 971 has had success with doing that (albeit with Ethernet and UDP broadcasting). You could also do some fancy off-board image processing as well. If you just need to tell your robot to turn right or left, then you could just transmit that information digitally through the MXP port.