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.
He won't be tried by UCMJ. If Brennan is tried by military tribunal, his sentence can be looked at by 3 judge panel. Not the USSC. UCMJ isn't applicable. LOAC controls this if they do it the way I think they will.