All you'll need to do is show up with a photo ID and take a written exam, but you will likely have trouble getting a photo ID without registering for selective service (if you're a male 18-25). That's how they got me, I refused to register because I believed the draft was unjust, but was pressured into getting a driver's license and had to sign my draft card to get it.
To be honest we've reached the end of my expertise but I think that would work. This thread from 2007 says it will work as long as you don't represent a foreign government.
