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I think you’re almost exactly right about what should be considered a “good” variable, but I’ll throw one suggested change at you: Instead of taking the log of the number of possible ontic *states*, what about the log of the number of possible ontic *histories*? This associates “entropy” with regions of spacetime rather than instantaneous slices of some system, so it’s a bit different than what we’re used to entropy-wise, but I think this has to be the right way to think about block-universe retrocausality, as I outline in the toy models in http://www.mdpi.com/2078-2489/5/1/190 .
This viewpoint also makes it natural to change the structure of the phase space, exactly as you describe. (Again, see the linked paper.) The 3D analog is finite-edge effects in stat. mech., where the structure of the border of the region changes the state space for which one calculates the probabilities inside the bulk. Now, extend this analysis to 4D, and you have a clear-cut way to allow the structure of the 4D border (including the future!) to change the state-space in the enclosed spacetime region.
As for how this might tie into my rotating-spin-vector, you can try wading through http://arxiv.org/abs/1301.7012 for some ideas, but there are plenty of unresolved issues here, many of which I’ve placed on the back burner while I’m working on partially-entangled states.