If PCLC turns off the green lights at the end of the race, then you should be able to to trigger the flag guy as well. Depending on what power your little guy runs at. If he requires less than 5v and less than 40ma, you should be able to run him off the existing Arduino, provided you have an open pin available. If he takes more power, then you will have to run them off a switch connected to your Arduino.
Either way, you will have to add some code to the PCLC Arduino code, but don’t let that intimidate you. If you know what pins you plug the green lights in to, you can search the PCLC code and find where those lights are turned off when the race is finished, then you could set whatever pin your guy is hooked up to to “HIGH”, which will make him waive his flag.
You inspired me, with your question about the blimp, and I went out and taped off the yellow section. Thanks for following up with me about it.