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Thanks, Andrés (and similar belated congratulations), this is great. In retrospect I had seen $\mathbb E$ and the book you mention, but at least the random part is news to me.
@Peter: In that case this question is still open, because about $\Omega$ you can prove many nontrivial things, e.g. that it is ML-random. But is it possible to define "nontrivial"?
@Peter: I'm sure Voevodsky had in mind something more like my answer than Rodrigo Freire's answer. But if we really want to talk about all the facts about $S$ being undecided then Rodrigo's answer is just as correct as mine (and simpler). Because for any finitely many $n_i$ one could make another definition of my column of $\Omega$ where the facts about whether $n_i\in$ that column are built into the definition.
Nice. Well, my more complicated set has the redeeming quality that no matter which first order definition of it you use, you can only prove finitely many facts of the form $n\in S$, $n\not\in S$.