Why not let everyone participate? If someone from outside the U.S. wins, then assign an attorney to look into it. If the prize or a modification of it can be awarded, award it; if not, don't. This way you don't blow money on attorney's fees to look into countries that in the end don't have any winners.
Of course, the contest rules would need to inform international entrants of their situation.
It seems ridiculous to have law firms making custom investigations for things like this. There should be (or may be) one or two companies that keep track of these rules and offer consulting on contest rule wording and prize selection for contests like this. Why reinvent the wheel for every contest?