Additional pass in surface shader (enable/disable pass)

Just write your additional pass after your surface shader code:

 Shader {

   SubShader {
 
     CGPROGRAM
       // Surface shader
     ENDCG
 
     Pass {
       CGPROGRAM
         // Additional Vert/Frag shader
       ENDCG
     }
   }
 }

You can add custom keywords to a shader using the #pragma directive:

 #pragma MY_KEYWORD

You can then add conditional code within the shader based on whether the keyword is defined, as:

 #ifdef MY_KEYWORD
 ...
 #endif

And you can change the keyword from script, using

 Shader.EnableKeyword("MY_KEYWORD");
 Shader.DisableKeyword("MY_KEYWORD");

(I've never personally tried enabling/disabling an entire pass like this, but I guess it would work).