Shaders: How to ztest against specific layers?

I made this shader to overlay a color when my player's mesh is hidden by other geometry in the scene:

 SubShader 
      { 
          Tags { "Queue"="Overlay+1" }
  
          Pass
          {
              ZWrite Off
              ZTest Greater
              Lighting Off
              Cull off
              Color [_ColorInvisible]
          }
  
          Pass 
          {
              ZTest Less
              Material {
                 Diffuse [_Color]
                 Ambient (0,0,0,0)
             }
              Lighting On
              SeparateSpecular On
              
              SetTexture [_MainTex] {combine texture * primary DOUBLE, texture * primary}
          } 
      }

The shader is working great but I'm unable to make it work only with specific layers:

I would like to make the overlay only when my player (wich has the above shader applied) is behind objects on a specific layer (i.e Wall layer).

How can i achieve this?

Thanks in advance.