Interior Mapping shader self shadowing
Solution 1:
Based on my very limited understanding of what you're trying to implement, it seems you would need to take the location of the intersection between the eye vector and the interior plane it hits, then trace it back to the light.
To trace back to the light, you would first have to check if the interior plane intersected by the eye vector is back-facing from the light's perspective, which would make it in shadow. If it's front-facing then you have to ray cast from within the room to the light and check for an intersection with any of the other interior planes.