I've seen statements of Newcomb-like problems saying things like "Omega gets it right 90% of the time". In that case it seems like it should matter whether it's because of cosmic rays that affect all predictions equally, or whether it's because he can only usefully predict the 90% of people who are easiest to predict, in which case if I'm not mistaken you can two-box if you're confident you're in the other 10%. I'm sure this would have been thought through somewhere before.

You may have misunderstood what is meant by "smart predictor".

The wiki entry does not say how Omega makes the prediction. Omega may be intelligent enough to be a smart predictor but Omega is also intelligent enough to be a dumb predictor. What matters is the method that Omega uses to generate the prediction. And whether the method of prediction causally connects Omega’s prediction back to the initial conditions that causally determine your choice.

Furthermore a significant part of the essay explains in detail why many of the assumptions associated with Omega are problematic.

Edited to add that on rereading I can see how the bit where I say, "It doesn’t state whether Omega is sufficiently smart." is a bit misleading. It should be read as a statement about the method of making the prediction not about Omega's intelligence.