Using substances on terrain

Our studio recently purchased Allegorithmic Substance Designer 2, and we are very pleased. However, I would really like to use a substance as a terrain texture (splat). At a tech demo recently put on at GDC Online, I recall them using substances on the terrain to accumulate snow over time (at least I think they were using the Unity terrain engine).

Does anybody have any suggestions as to how we might go about creating and using a substance as a terrain texture in Unity?

We just released a free tool that does just that :
http://forum.unity3d.com/threads/109750-Substance-Terrain-Tool?highlight=substance