I looked at the Fluke 720A manual, and the only way I can see the load impedance of a DMM in ACV range not destroy any accuracy, even at very low frequencies, is to use the divider at ratios like \$2\cdot10^{-4}\$ and lower. That way the output impedance is low enough that variations in the DMMs input impedance will not swamp the uncertainty budget, but uncertainty due to the KVD linearity of 0.1 ppm of input will be 0.05%, and you will still need to correct for the loading impedance. The lower the ratio, the higher the linearity error and the lower the loading error. So I'd argue KVDs like this are not useful for AC unless you can increase the load impedance using either potentiometric technique (null meter against another divider) or some sort of buffer amplifier.
If the only usable way to use a KVD for ACV is using a null meter because the loading due to the input impedance of a DMM is too high, then I'd argue that fact alone is enough to severely limit their usability for ACV. Since you'd probably need to determine correction coefficients for the KVD for ACV using another divider, what would be the point of subsequently using that KVD to compare it to yet a third divider?
I don't see how an AC null meter would work with another non phase locked source like adjusting an ACV calibrator or stable function generator to a Fluke 510A.