This is very interesting! I did see a post somewhere on this awesome forum from either Matt or Oshyan about the possibility/plans of eventually supporting Substances.
The way substances work in 3D applications & 3D Game Engines that support the .sbsar file type (3DS Max, Modo, Unity, Unreal etc...) is they are built/baked at render time at the specified resolution, so really the user could specify whatever resolution they need for the rendered image to look good. This would be awesome from a materials/surfaces standpoint, because you wouldn't have to bake out your substances into bitmaps if Terragen supported the .sbsar file type in some way.
I am not a programmer, but I would imagine that like some of the other File Loader nodes in Terragen, there could be a Substance Loader node, which could be used like a bitmap, but have a panel of variables that could be changed through the use of sliders like many TG nodes. These variables would slightly alter the material similarly to how you can in Substance Painter.
If something like this were to happen, you would be in "Node Heaven" using TG and Substance Designer in combination. Dreams would be born, and other dreams would come true.
Oh, and I finally can spell Oshyan without looking at a reference.