I've just arrived back home after being in the US for some months. I was required to have a FAA class 3 medical, because I did some flight training there, and luckily the one I've received is also is a student pilot certificate with the number on it you require to register for DUAT. Some of my colleges didn't have this number and so they were not able to register until getting there FAA pilot certificate. This will be send to your home adress, most probably when you are home again. In the US you'll only get a temporary certificate (peace of paper). The only number you'll find on it is your current license number.
Now what you can try: One of my colleges simply wrote an e-mail to duats helpdesk where he described his situation, giving all the information they require on their registration page with his foreign license number and attached a photography of his license and a proof that he'll be in the us for the next months. Less than 24 hours later he had a log-in. We had a student Visa, but if you don't have something like this, just try it without it, or take a photography of something else looking trustworthy.
Good luck and enjoy your trip!
Edit: I was just looking for the e-mail address, but for any reason my browser refuses to load duats website. As far as I remember, you'll find it at the registration page. If someonewho can access the website likes to add the e-mail adress please feel free to do so, I would appreciate your help.