You might want to look at a 1-wire Counter as an interface to your CarPC.
You could add it to the power feed for both your ignition switch and another on your headlights. Maxim is the company that makes the chips. I think I recently saw a company that made both kit and assembled counters for like $25.
You can't sniff the lights with OBD.
There is a count of number of starts since last fault or something like that that might help with number of starts. Still would be kludgy.