Can a US citizen be tried by military tribunal?
Jonah Goldberg says yes in this commentary in today's Washingtion Times:

The issue of whether American citizens could be treated as prisoners of war was settled six decades ago in Ex Parte Quirin. The Supreme Court ruled that sneaking into the United States with the intent to destroy "life or property" is an offense "against the law of war subject to trial and punishment by military tribunals."

The case dealt with eight German agents who infiltrated the United States intending to blow up factories and disrupt transportation. Two of the agents were American citizens. Six infiltrators were executed, including one of the Americans. In response, the court ruled that being an American citizen "does not relieve" you from the obligations of the rules of war. "Citizens who associate themselves with the military arm of the enemy government, and with its aid, guidance and direction enter this country bent on hostile acts, are enemy belligerents within the meaning of the Hague Convention and the law of war," the court concluded.

That seems pretty clear.