No, the variance in each bin is not a new predictor, but would combine with the variance in the dependent variables to give a weight in the chisquare sum (in the way which I indicated above, maybe there should be a square root). This seems intuitively true, but not sure how rigorous it is as far as obtaining true parameters from the limit of infinite data sets. Is there a Mathematica function or option to LinearModelFit that can do this?