Wang set with "empty" color?

Is there any way to create wang set with one color “empty”
In terrain you can mark terrain like that:

sides that are not marked as terrain are automatically treated as connector to empty neighbor.

Is there any way to do something like that with wang sets? It would allow to draw walls and paths easily. Or terrain types that contains one-tile width paths as well.

Currently, no ): It would be great to be able to include “none” as a colour, or to mark the theoretical empty tile with wang colours, but it’s not currently possible.

Then Bjorn please consider this as feature request.