I don't think it's a bad idea but it brings some complications:
how many times we decide not to post a car because that will be really too hard? There are a lot of cars that will be impossible, really impossible, to find, unless someone has the same book, magazine or old picture. If we decide that all the 1-year old quizzes will award a point for its author, I can imagine an increase of such cars! Sometimes the quizz is just the detail of a car. If someone has the great idea to post a really insignificant detail waiting for the points? I know it's ridiculos to act like this, but if we create the opportunity, then we can't avoid that.
Sometimes I start from the last unsolved puzzle and go back to the newest. Many times I'm surprised to find out a car I know, other times I just bump it in the first page and someone else solve it. There are too many new puzzles each time, so the attention goes to them, but the old ones are not so hard if we concentrate on them.
I'm sure that if we start to ask for clues for some cars, we can find them. I posted a really obscure car, the Burrowes, remained for months without a single guess. Then i wrote "..it was american and if you have your cars alphabetically listed, you can avoid to look in the last part of your list...." and in few days Ray B. found it.
Not a bad idea, but it needs some rules.