Quote from Bob Smith :

If you are to release your source to other developers, I would advise against doing so publically. The last thing we need is the licence checking system exposed to tech savvy demo racers.

I both agree and disagree whit you. I think the source code shall be released, but i also understand why you don't want to.
Here is two ways it might been done painless:

1. In the next version, add some kind of debugmode or somthing so the player by himself can tweak the car.
2. Make it possible for the demoracers to use the tweakprogram to. But only whit the democars. And we paying users would have the tweak for all the cars.