Earlier today I was a bit bored and decided to try to get a ship to move backwards instead of forwards, without much success.
Just setting the ship's acceleration and speed to negative numbers didn't work, but I expected that to be the case.
I also tried giving ships an outfit with a negative value in the speed/acceleration fields, which would give the ships less than 0 accel/speed. This didn't seem to work, either. (In fact, the game seemed to totally ignore the outfit.)
Anyone else have any other creative ideas I might try?