If you're prepared to use OSL, (and therefore the CPU for rendering,) this little script might be a tool to add to your box:

#include "stdosl.h"

shader LitObjectIDX(  
         output float ObjectIdx = 0.0        
)
{  

    if (trace(P,I)){
        getmessage("trace", "object:index", ObjectIdx);
    }
}

It casts a ray back down the incoming direction, and returns information about the object that it finds there. In this case, it yields the Pass Index of that object. It can be used in a shader for a light to modify the light's behaviour according to the object it's lighting.

In this example, a screen is erected across the scene...

... with a gap in it for the door.

The screen is made of an ordinary fully-transparent BSDF, with 'Transparent Shadows' on, so it's invisible to most lights. But, if we give the object a Pass Index of, say, 20, It can be persuaded to cast shadows for any light with these nodes:

In this case the Transparent Depth (>0) also ensures we can't see the effects of the light seen through the screen.

So, when the yellow light uses the nodes, the overall effect can be this:

This is limited: taking the camera behind the screen gives the game away, but you might be able to use the script in ways more precisely geared to the effect you're looking for.

(This answer revised thanks to @lemon pointing out what should have been obvious..)

If you're prepared to use OSL, (and therefore the CPU for rendering,) this script might be a tool to add to your box. By tracing back down the incoming ray to a light, it returns information about the object being lit. It's not all used in this example, but might be useful if you go for a different effect.

#include "stdosl.h"

shader LitObjectIDX(  
        output float    ObjectIdx = 0.0,
        output float    MaterialIdx = 0.0,
        output vector   LitP = 0.0,
        output vector   LitN = 0.0        
)
{  

    if (trace(P,I)){
        getmessage("trace", "object:index", ObjectIdx);
        getmessage("trace", "material:index", MaterialIdx);
        getmessage("trace", "P", LitP);
        getmessage("trace", "N", LitN);  
    }
}

ObjectIdx and MaterialIdx are the Pass Index settings for the object and material being lit. LitP and LitN are the location and the normal of the lit point, in World space.

Make a 'Portal' plane, of ordinary fully transparent material, occupying the doorway. For this tree, give it a Pass Index of 20 in its Object > Relations panel. You could use other indices, for other portals.

This node set-up for the yellow through-light restricts the lighting to the portal itself, and areas seen through the portal, contrasting those seen from behind and those seen from in front. Since writing, you've told me that the camera doesn't need to move, so the second restriction, (most of the bottom branch) is not required. Just use the Transparent Depth on its own. My effect looks a bit odd, anyway.

The attached .blend is a bit more complex than needed just to illustrate this method.. you would probably just want to use the script, one way or another, to suit your particular purposes.

If you're prepared to use OSL, (and therefore the CPU for rendering,) this little script might be a tool to add to your box:

#include "stdosl.h"

shader LitObjectIDX(  
         output float ObjectIdx = 0.0        
)
{  

    if (trace(P,I)){
        getmessage("trace", "object:index", ObjectIdx);
    }
}

It casts a ray back down the incoming direction, and returns information about the object that it finds there. In this case, it yields the Pass Index of that object. It can be used in a shader for a light to modify the light's behaviour according to the object it's lighting.

In this example, a screen is erected across the scene...

... with a gap in it for the door.

The screen is made of an ordinary fully-transparent BSDF, with 'Transparent Shadows' on, so it's invisible to most lights. But, if we give it a Pass Index of, say, 20, It can be persuaded to cast shadows for any light with these nodes:

In this case the Transparent Depth (>0) also ensures we can't see the effects of the light seen through the screen.

So, when the yellow light uses the nodes, the overall effect can be this:

This is limited: taking the camera behind the screen gives the game away, but you might be able to use the script in ways more precisely geared to the effect you're looking for.

If you're prepared to use OSL, (and therefore the CPU for rendering,) this little script might be a tool to add to your box:

#include "stdosl.h"

shader LitObjectIDX(  
         output float ObjectIdx = 0.0        
)
{  

    if (trace(P,I)){
        getmessage("trace", "object:index", ObjectIdx);
    }
}

It casts a ray back down the incoming direction, and returns information about the object that it finds there. In this case, it yields the Pass Index of that object. It can be used in a shader for a light to modify the light's behaviour according to the object it's lighting.

In this example, a screen is erected across the scene...

... with a gap in it for the door.

The screen is made of an ordinary fully-transparent BSDF, with 'Transparent Shadows' on, so it's invisible to most lights. But, if we give the object a Pass Index of, say, 20, It can be persuaded to cast shadows for any light with these nodes:

In this case the Transparent Depth (>0) also ensures we can't see the effects of the light seen through the screen.

So, when the yellow light uses the nodes, the overall effect can be this:

This is limited: taking the camera behind the screen gives the game away, but you might be able to use the script in ways more precisely geared to the effect you're looking for.