There is an infinite width adjustment already available, at no cost, by the way. Just move off your mark the amount you want to widen and re-cut.

That's the beauty of the Domino. The cutter doesn't care if the material is solid or not. It's primary mode of removing material is side loaded milling, so if you're cutting a little air and just bumping one mortise end a little to lengthen the mortise it's no big deal. If you are milling a slot on a Bridgeport the equivalent here would be just cranking the handle a little further at the end you want to lengthen.

True, John, it is always possible to have a varying result when re-cutting. But, that begs the question: Which cut was the miscut? The first one? The second one? Both? I have generally found that re-cutting might make the mortise a bit looser but not objectionably so. Given the oft heard complaints that the dominoes are sometimes too tight I don't think it is much to worry about. In fact, re-cutting is a good way to self-check your technique. If I can make a cut, remove and reset the machine, cut again and still have tight fitting dominoes, then my technique must be good.

You were right on about not changing the width setting. The added benefit of using a new positioning of the mark is that you should know pretty closely just how much you are lengthening the mortise. I don't know how you would do that with a cam adjustment. In fact, there is a scale on the fence that can be used to offset precise amounts from the original center line.