Predicting with 99.99% accuracy that any person, put in front of the dilemma of tasting a pleasant cake or receive a kick in the teeth (or, to stay in the Portal metaphor, to be burned alive) will chose the cake, is clearly not relevant to the free will debate.
Is there an existing principled explanation for why this is not relevant to the free will debate, but predicting less obvious behaviors is?
Because any working system evolved from self-preservation would do that. It doesn't add any bit of information, although it's a prediction that has striking accuracy.
Scott Aaronson has a new 85 page essay up, titled "The Ghost in the Quantum Turing Machine". (Abstract here.) In Section 2.11 (Singulatarianism) he explicitly mentions Eliezer as an influence. But that's just a starting point, and he then moves in a direction that's very far from any kind of LW consensus. Among other things, he suggests that a crucial qualitative difference between a person and a digital upload is that the laws of physics prohibit making perfect copies of a person. Personally, I find the arguments completely unconvincing, but Aaronson is always thought-provoking and fun to read, and this is a good excuse to read about things like (I quote the abstract) "the No-Cloning Theorem, the measurement problem, decoherence, chaos, the arrow of time, the holographic principle, Newcomb's paradox, Boltzmann brains, algorithmic information theory, and the Common Prior Assumption". This is not just a shopping list of buzzwords, these are all important components of the author's main argument. It unfortunately still seems weak to me, but the time spent reading it is not wasted at all.