How about a camera which attaches to the cycle with built in GPS (for location and speed data) and some sensor to detect how close a passing car is (ultrasonic?)
then, if a car passes too close it automatically grabs a picture with the cars number plate and this along with info on how fast the cycle was going, how fast the car passed and how close it was, this data can be uploaded later via the internet to a central database.
Then a letter could be sent to the driver explaining how their actions cause fear and danger to cyclists etc. etc. asking them to be more considerate.
if the same vehicle is flagging up on a regular basis from several cyclists then maybe the police could be asked to take action?
Commuting, Day rides, Audax, Incidents, etc.