Since I just read through the functions (again, not actually able to code):
The key doesn't actually connect to the car, your player does. The key is just what executes the functions when you use it.
I'd assume the mod could be rewritten to bild the function to the key, but that raises another problem I think. Look at factorissimo with creative mode installed. There's not really three factory buildings, there's dozens of them with the same visible name. You only ever get to craft one. However I think that each building is actually tied to the layer it creates.
Back to the keys, that's mean there would have to be several dozen keys in the game (and eventually, you have them all and can't get more) so that each can tie to a vehicle. Doable I guess but don't ask me how.
