I think that would be hard to implement. Basically, vehicles are tied to a player, not a key. The key is just a tool to make that connection. Having different keys for different vehicles would mean the keys would have to have a stack size of 1, so you'd waste inventory spaces if you'd have several keys.
Also, what would happen if you lost your keys? Perhaps you drop them accidentally, or forget them in a chest, or you die and can't find your corpse in time before it expires. What should happen in that case? You wouldn't be able to craft another key and connect it to your old car again.
Also, while I can see that being able to claim different vehicles would be quite useful in single player mode, I'm afraid this feature could be abused in multiplayer if griefers start running around and start claiming all vehicles.
This is one of the things that look quite easy to implement but really are complicated. Before I'd ever start on implementing anything like that, I'd have to give a lot of thought to it because it would drastically change how the mod works. The next best thing would be a list of all the vehicles connected to a player (either because they are the currently "owned" vehicle or because they've been locked) so that you'd be able to call any vehicle at any time. It's not really plausible because a locked vehicle is not operable, not minable, and can't be opened -- so it would be hard to explain how it would come to you. Also, what would happen to the vehicle you currently own if you call one of your locked ones? If you forget to lock it, anybody could take it.
Several players already mentioned that they find the way GCKI works too cheaty (you'll find some of these voices over at the Autodrive discussion board). Naturally, I'm quite biased, so I don't share that sentiment -- but that would be different if you could control different vehicles at the same time. Making that optional, and only for single player games, may be different -- but again, there's a lot of cases to consider. I'm glad it's working now -- and I'm quite afraid of having to add even more code to the mod's logic.