I have heard this explanation before but have to admit that I don't understand it. Regardless of how many names are on the list, you should be able to fill customer spec'd cars in the order of the list. I agree that factory spec'd cars go to the first person you can contact who wants it but at least everyone knows the longest they will have to wait based on where they are on your list. I would assume that after the first month or two, almost all cars will be customer spec'd and the customers will know a long time in advance when their cars are going to arrive.
You should be able to get your customer's requirements before they need to be sent to the factory - I can give you mine right now! I don't see how you get in to a situation where a customer taking a few days to get back to you causes problems. If they take weeks to return your call then they deserve to lose their place in line.
Is it more complex than this - am I missing something?