Good idea. But, for now, the existing "Community races" can be used just for the purpose.
You can just pick (or duplicate) one of the races from the ones provided. Then, enter the game, get the checkpoints coords and enter them to a respective .xml file. Voila, you have your own circuit with a finish line counting as a time trial marker.
But I get the point - you want a tool where you can automatically save checkpoints "on the fly" without text editing every damn number. Also, I'm afraid "Community races" does not allow MP and add-on cars as racing vehicles.