You only need up to 4 vertices to define any mapping that can be defined in SketchUp. Just use UVs from any 4 non-collinear vertices.
Here’s an example C++ project that maps a texture to a face with 6 vertices. Note that I’m using only 3 UV coordinates.
TextureMapping.zip (89.8 KB)
You can extract this into the SDK package under samples/C++ and it should compile.
Here’s the face it generates:
Let me know if you have questions on the code.