You may already be aware of these, but there's a vendor at DAZ who has some great Photoshop brushes that can help a lot in making clothes muddy. He's called "Deviney". Search him up.
You can use them to paint mud splatter, splashes, coverage to your heart's content on either the clothing's texture files or on the finished render itself. If you do it on the texture files, you'll be able to do multi-frame series with nice consistency of appearance.
Some of the ones I use are called: "Ron's Mud", "Ron's Sand", "Ron's Stained, Splatter, Drips, And Wash", etc.
There are also excellent wave and water brushes for splash FX. Hope that helps.
