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I think it would be really cool if I could come up with formulas for those and maybe even provide a pose file that would establish some ERC magic to set the limits for, say, the shoulder bend channel, as a function of the current values of all the other channels. But that's pretty much a pipe dream at this point.

Been thinking about the same, but concluded it would need to be in the form of 'posing by command'. The parameter space you are looking at is likely concave in some areas, and 'posing by dial' would fail because users do not know the perimeters of the allowable parameter space. 
Posing by command ('left arm forward', 'lift shoulder') would probably be a way to solve that.

But it would be certainly beyond my reach.