Would the most useful response in this real-world situation be: 1. Take both boxes. 2. Take box A. 3. walk away, not taking any money, because you don't understand his wacky game and want no part of it, before going home to hit the Internet and tell Reddit and LessWrong about it. 4. Invent timeless decision theory?

I think the first thing I would do is ask what he'll give me if he is wrong. ;-)

(Rationale: my first expectation is that Mr. O'Mega likes to mess with people's minds, and sees it as worth risking $1000 just to watch people take an empty box and get pissed at him, or to crow in delight when you take both boxes and feel like you lost a million that you never would've had in the first place. So, absent any independent way to determine the odds that he's playing the game sincerely, I at least want a consolation prize for the "he's just messing with me" case.)