Cletus, perhaps we are conflating courts martial with military tribunals - two different things - for two different purposes. Similar process to a point, but with far, far less review/appeal authority as the defendants are enemy combatants instead of citizens or lawful enlistees.
So, where do we land? If Brennan is found guilty of violations of UCMJ in a duly convened tribunal, can he appeal that verdict to SCOTUS? I say no. Can he appeal the process (trying a civilian in a military tribunal under UCMJ)? I think so as long as he sues for certiorari/habeas corpus before the tribunal begins.
SCOTUS can hold up such a tribunal for them to render a decision.