For any decision theory, isn't there some hypothetical where Omega can say, "I've analyzed your decision theory, and I'm giving you proposition X, such that if you act the way your decision theory believes is optimal, you will lose?" The "Omega scans your brain and tortures you if you're too rational" would be an obvious example of this.

This isn't obvious. In particular, note that your "obvious example" violates the basic assumption all these attempts at a decision theory are using, that the payoff depends only on your choice and not how you arrived at it. Of course this is not necessarily a realistic assumption, but that is, IINM, the problem they're trying to solve.