That’s the only use I’ve ever seen, and it feels like an allusory use to me: similar to “precious cargo”. I have never noticed anyone using “on board a car” aloud, but it could be a regional thing I suppose.

Ive heard phases like “all aboard!” and “permission to come aboard?” that are specifically meant to jokingly evoke train and ship travel, but never in a matter-of-fact way like “he spilled his coffee aboard his car.” or “she was onboard her sister’s car when the deer jumped out in front of them.”