What type of application are you referring to here? A camera that would be used to record accidents, or ones that are used for in-car video during track events? If during track events, most of those are started shortly before the car gets on the track, they continue to record for the entire session, then the video is downloaded and post-processed.
If you're referring to an application that would capture accidents, the cameras could be triggered by the brake pressure switch that some vehicles have, the TPS or WOT discrete, and/or airbag trigger, or as you mention, various g-sensors within the car could provide that as well. But I'm not aware of any cars that come standard with such dash-mounted cameras and these discrete signals may not be readily accessible.
-Rod