Here is a solution: copy Apple's structure for the Apple Design Awards which are after all international. Apple has good laywers, you can trust they did a good job.
Secondly, I personally will never enter any contest that sets international developers apart from US developers. US Developers maye get a price, the other may get 'recognition' and will be allowed to 'brag'? Come on, how do you think it feels to be treated like a 2nd class participant?
I personally think that as long as you are governed by lawyer-'angst' it is better to rename the contest the "National O'Reilly etc" to make it clear it is a national, not an international contest. And remove the 'country' field in your form while you're at it.
It is one of the most frustrating experiences to be discriminated against, just because one is outside the US, especially when it is something that has nothing to do with nationality. Probably the majority of my users are in the US, the software is only in English even.
Incompetent lawyers (who cannot even copy one of the existing international contests) who guide weak and fearful managers is what characterizes O'Reilly here. Is your lawyer making you afraid? Then he is probably incompetent because he *should* provide you with a solution. Take a better one and look at it from a managerial point for a change. (And yes, I do have international legal experience).