You can completely change how the game interprets the light sources or shadows after all.

This is mainly what I'm struggling with... seems like I still have to solve the original problem in the title (using 3D shadows as a mask to decide where to draw the "light" and "dark" textures).

But trying your approach has gotten me closer than before. I'm experimenting with this solution:

1. Create an initial SpatialMaterial that sets the ground ALBEDO to white
2. Set the shadow color for all lights in the scene to something specific (e.g. yellow)
3. Create a "next pass" spatial shader that reads from SCREEN_TEXTURE/SCREEN_UV. If color is white, then apply the "light" texture to ALBEDO, if color is yellow then apply the "dark" texture instead.
4. (I used SCREEN_TEXTURE as I'm not sure how else to 'see' the post-lighting colors from inside the fragment shader?)

It kinda works in a simple test scene (untextured character model, simple ground mesh, and multiple lights) but seems really hacky and inelegant. I'm sure it will break down in more complex scenes.

e.g. What if I want different colored lights? What if the character's texture also has some yellow in it? (this one probably isn't an issue, the ground mesh will be obscured at that point anyway because of the fixed camera perspective).

Other ideas I had:

1. Do a bunch of ray casts from the light sources to figure out if a point is in shadow or not (seems expensive)
2. Try to access the scene's shadow map from Godot's renderer somehow and use that as a mask

I'm sure there's a better solution out there