Can a civilian be tried in a military tribunal?
The U.S. Supreme Court agreed, and unanimously ruled that military tribunals used to try civilians in any jurisdiction where the civil courts were functioning were unconstitutional, with its decision in Ex parte Milligan (1866).
Now the logical inverse question would be: What if the civil court in that jurisdiction were not functioning normally? (i.e. under fire from insurrectionists)
I haven't found the definitive answer yet, but logically, my previous answer would make sense.