Texture sampler address modes - border colour

How do I set the 'Border' address mode equivalent to dx11 border mode?

https://msdn.microsoft.com/en-us/library/windows/desktop/ff476256(v=vs.85).aspx

As opposed to wrap, mirror or clamp, this would allow me to set a colour to be used outside of the uv boundaries. I'm using the projector shader to project one camera's view on a mesh, and want the colour of any part of the mesh that isn't projected on, to be black

 

I suspect it is not supported, just like it's not possible to make a sampler state with anisotropic filtering enabled.

 

It's fairly widely available, so it would be a decent feature request towards Unity.
https://msdn.microsoft.com/en-us/library/windows/desktop/bb172363(v=vs.85).aspx
https://open.gl/textures