I was actually wondering about this as well, I almost tried to fix it sometime last year -- if you can find the source for the RDS home applet, I think it can send whatever data it wants, you can send the text "I'M WATCHING YOU" if you wanted to neighboring cars if the signal was strong enough. Just need to use whatever API is available for it. :P
echo -n IM WATCHING YOU > /sys/class/i2c-adapter/i2c-2/2-0063/rds_radio_text