I am attempting to make an environment that has hand-painted shadows in the base color texture. This would give me more artistic control over how the shadows are cast. However, I would also like shadows to be cast by moving/animated objects that blend seamlessly with the painted environment shadows.
A good example can be seen in the gif below:
Here, the artist painted in the static shadow on the left while the box is casting a realtime shadow in Unity.
Based on his explanation here, I would imagine this could be achieved by creating an emissive base color texture with a dark/shaded version revealed by a hand-painted mask. At the same time, shadows cast by other objects could be used as additional masks to reveal the dark/shaded texture, allowing them all to blend seamlessly.
Unfortunately, I am at a complete loss as to how this might be achieved and have yet to find anybody else do the same. Is this sort of shader setup possible within Blender?
EDIT: I am using Cycles for this case.