Comparison between shader V1 and V2 (click to enlarge).

It turns out that the hair has seven or eight different mat areas and I only applied the mud shader to one of them, which is why it didn't show up in my first renders.

Also, I think the reason why the mud displacement map didn't show up in the test render is that I lowered the contrast to counteract the lack of VSS to match the skin tone a little better, but that made the texture of the mud less visible.

Regardless, I much prefer the way V2 looks.