Adam, yes, use a third texture in C channel cause it is governed by the world scale otherwise. (As with A and B, Ctrl+Shift+click on C.) I suggest either using one of the fractal textures - Basic Altitude, note the capital A, and it's the second one of the fractal Basic textures - or using a gradient created in Photoshop. White means the texture in channel B will be applied and black means channel A. With the fractal texture you can use World Space mapping or World Side. You can adjust the ratio of mud/moss by going into Deep Texture Editor and adjusting the values in Filter dialog. Interestingly, Object Front mapping also works well, while Object Side doesn't. I don't have an image gradient but you will adjust the ratio by changing the white/black point in Photoshop and trying the mapping of the third channel. If I was unclear, just yell.

-- erlik