I try to only answer questions about Castle products on here instead of actively promoting our products, but I think Castle has a really good solution for controlling the speed of your pacer.

We have a communications protocol we call Link Live built into all of our ESCs. That allows telemetry feedback from the ESC to the receiver via the normal signal wire. It is a very strange protocol and kind of hard to implement, but it works really well. I know there is a library out there for using it on Arduino, but I don't think know of any for R-Pi. We sell an additional device called the serial link that handles all of that weirdness and converts it to a more traditional serial or i2c communication. The serial link can receive the link Live data and broadcast it back via serial, but it can also control the throttle via the same serial so you don't have to have your controller outputting a PWM pulse constantly.

When I think about the point of a pacer, I think of something with a very controlled speed. With a normal RC control, setting to 50% throttle might be the speed you want at first. As the battery drains, 50% throttle is no longer the speed it was going on a full charge. With Link Live, you could get feedback of the exact motor RPM 11 times per second and then adjust your throttle input very slightly to maintain a a very specific RPM. If you have a Castle ESC that has datalogging(like the Mamba X) you could also get an exact feedback of the amount of current used by the ESC/Motor so that you might be able detect object collisions by detecting when current goes up and RPM goes down. You could also use the motor RPM to calculate exactly how far you have gone.

Sorry for the promotion back to the project. I think if you had a pair of 7 segment displays showing on the back of the car that were showing the current speed it would be cool. Also instead of using an r/c transmitter to control, I was thinking you should get an IR receiver that the PI could listen to and then use a tv remote to control it. Power button as start/stop, and you can enter a channel number as a target speed. So if you type 50, then it will try to maintain 5.0 mph. Channel up could increase it to 5.1mph, channel down would change it back to 5.0 mph.

Another random/fun idea, put a sensor on the back to detect how far you are from it and if you ever start falling back have it play something motivational like Eye of the Tiger or the Rocky theme song over a speaker.