I have trouble imagining a scenario where he could introduce conclusive evidence in either case because, just like objective value, either example would still include the disagreement involved in his original argument which makes them require consistency.  His most likely response would be to utilize the same strategy and infer here that the best explanation is they are different cases altogether and require different lines of reasoning. This however simply adds to the inconsistency in which he uses that line of reasoning and does not solve the problem.  Rather it strengthens some convictions I have been attempting to develop between the lines about the nature of inference to the best explanation as a form of argument. 
The lack of evidence needed to infer the best explanation by Mackie’s standards frankly frightens me and shows its true ineffectiveness.  How can correlation between disagreement and objective truth be simply inferred without conclusive evidence? Because it is the easiest answer? That does not seem at all satisfactory to me; that reasoning is flawed and can not conclude anything with certainty.  It is as if we are simply accepting the wrong answer for now until new information and evidence comes to light instead of searching for that evidence. This makes inference to the best explanation seem distinctly inappropriate reasoning in any case of disagreement.  Why then does Mackie instinctually pair the two (as he has no real evidence that this is appropriate)? There must be some relation between the two that caused this association and I believe the commonality lies within the flaws already laid out against inference to the best explanation.  What this says about disagreement is that it is a form of argument that lacks enough evidence to make any remark concerning objective moral values. In fact I believe that the two can not even associate with one another.
Disagreement is incorrectly defined by Mackie and others as something that is objective because how could it say anything about objective truth if it was subjective? The answer here is that it can not.  Disagreement is wholly subjective to the human condition; it is a facet of our nature. We do not only disagree about morality, we disagree about non-moral fact so disagreement is not something specific to morality, rather it is specific to humanity. It takes on different forms, for example compromise.  And how could we compromise about objective value?  Mackie would say this only alludes to the fact that there is no objective value, but the true answer here is that disagreement lacks the ability to comment on things objective in their nature. Would it not only then be natural for us to disagree about morality regardless of its objective nature? It is a mistake to link the two because they are completely separate and lack the ability to provide insight about one another; disagreement is subjective in its relation to humanity and therefore can hold no sway over the truth or falsity of objective value. 