It's a good idea, in theory. It's been attmepted many times before, at many, many different forums.
However, it's doomed to fail here. These forums do not have a sort of "Sticky" option, that can hold the topic up at the top of the page. And so, as long as this topic stays on it's original topic, it will eventually fall down the page.
Sure, you could bump it up, but that's no better than spam.
Another problem is, even at sites that do have such options, New Members often ignore this post anyway. A lot of New Members come once, and leave, as you said. It happens a lot. However, I'm willing to bet, most of them don't even look at the topics before posting.
I'm sure this may save us a few threads, but in the long run, I don't think it will work out as well as you think it will.
