My answer to Newcomb's problem is to one-box if and only if Omega is not defeatable and two-box in a way that defeats Omega otherwise

But now you've laid out your decision-making process, so all Omega needs to do now is to predict whether you think he's defeatable. ;-)

In general, I expect Omega could actually be implemented just by being able to tell whether somebody is likely to overthink the problem, and if so, predict they will two-box. That might be sufficient to get better-than-chance predictions.

To put it yet another way: if you're trying to outsmart Omega, that means you're trying to figure out a rationalization that will let you two-box... which means Omega should predict you'll two-box. ;-)