First and fore-most ,I do not own these shaders.
With that out of the way , I found an almost working terrain hole shader here
it works fine and makes good looking holes when pared with the depth mask shader (on the same blog), but passed the fourth textuer you add to your terrain all the textures are black.
Is there a fix ? , I tried to contact the maker of the shader but couldnt.
I have found other terrain shaders that can have more that four textures like this one.
Textures 1-4 rendered with FirstPassShader
Textures 5-8 rendered with AddPassShader (if necessary)
Textures 9-12 rendered with AddPassShader (if necessary)
etc…
.
conclusion: You need to edit the RenderQueue of AddPassShader too.