How do you set the UVs for a decal texture?

Hey guys. I'm procedurally-generating a 2D tile-based world by generating large sub-divided quads and assigning the UVs of each subdivision to a tile in a texture atlas. Which I currently have working like a treat.

Now I'm up to the tricky part. The idea is to use the decal shader to create terrain details like the edge between different terrain types. For example, for every non-sand tile I'd want to give it set its decal UVs to map to one of these -

The thing I'm stumped on is - how can I give the Decal texture a separate set a UVs from the Main texture seeing as the main texture's UVs seem to be based on the mesh's geometry?