Hmm you may be right...it may be during drop/add that you can waitlist...although if I'm not mistaken as well, you need approval to take over 4.5 credits as a freshman as well? Check your ACES rules, because one/both of us may be wrong!
If there are only 2 or 3 students on the waitlist now, after all upperclassmen have registered, I wouldn't see it ballooning to much more than that, to be honest. Register for 4 classes, email the professor a few days after registration or something like that if you can't add it. If he doesn't reply, take care of it when you get on campus in the fall. The first week of classes is basically like a "trial period" at Duke anyway...people always coming and going!