What do you mean unlock? Just because they have a big S or no picture at all doesn't mean it is locked.
A lot of them are actually for Special Edition and so on, such as the Khamelion and whatever the Hot Rod is. I don't believe you'll be able to unlock those unless they plan to make pre-order DLC a separate DLC. I wouldn't doubt it being released later in
a DLC - Look at Red Dead and the Hunter/DeadEye/Trader outfits and gold guns.
Edit: I believe the Elegy is also a locked car before signing up for the Social Club. I could be wrong however..