Would this be a negative mark on my credit, "paid in full for less than full amount due"?

If reported as such to credit rating agencies, yes, it would be a negative mark. If is not as negative as defaulting entirely, of course--most impacts on credit rating are not black and white, but rather exist along a continuum from full payment on time (obviously the best) to complete default (the worst). Partial payment in settlement of a claim would be somewhere between the two, so it would be negative if reported, but not as strongly so as other outcomes.