Sometimes a player really shouldn't be leaving a vehicle accidentally, like in a temporary space a vehicle might pass through where no player should be walking...
Okay, after taking a closer look, it's possible to trigger the enter-vehicle action but not unambiguously using player.driving ... Interesting.
That might be enough, but it would be nice to be able to save/restore the particular vehicle involved. Perhaps on_player_driving_state_changed could include the vehicle in question, whether entering or exiting--and player.vehicle could be made RW on top of the current player.driving?