Hiding parts of a mesh

If I understand you correctly, try this shader:

 Shader "Matte" {
     Properties {
     }
     
     SubShader {
         Tags {
             "Queue"="Background"
             "IgnoreProjector"="False"
             "RenderType"="Opaque"
         }

         
         Cull Off
         ZWrite On
         ZTest LEqual
         ColorMask RGBA
         Blend One One
         Fog {
         }


         CGPROGRAM
         #pragma surface surf BlinnPhongEditor  vertex:vert
         #pragma target 2.0



             struct EditorSurfaceOutput {
                 half3 Albedo;
                 half3 Normal;
                 half3 Emission;
                 half3 Gloss;
                 half Specular;
                 half Alpha;
                 half4 Custom;
             };
             
             inline half4 LightingBlinnPhongEditor_PrePass (EditorSurfaceOutput s, half4 light) {
                 half3 spec = light.a * s.Gloss;
                 half4 c;
                 c.rgb = (s.Albedo * light.rgb + light.rgb * spec);
                 c.a = s.Alpha;
                 return c;
             }

             inline half4 LightingBlinnPhongEditor (EditorSurfaceOutput s, half3 lightDir, half3 viewDir, half atten) {
                 half3 h = normalize (lightDir + viewDir);
                 
                 half diff = max (0, dot ( lightDir, s.Normal ));
                 
                 float nh = max (0, dot (s.Normal, h));
                 float spec = pow (nh, s.Specular*128.0);
                 
                 half4 res;
                 res.rgb = _LightColor0.rgb * diff;
                 res.w = spec * Luminance (_LightColor0.rgb);
                 res *= atten * 2.0;

                 return LightingBlinnPhongEditor_PrePass( s, res );
             }
             
             struct Input {
                 float4 color : COLOR;
             };

             void vert (inout appdata_full v, out Input o) {
             float4 VertexOutputMaster0_0_NoInput = float4(0,0,0,0);
             float4 VertexOutputMaster0_1_NoInput = float4(0,0,0,0);
             float4 VertexOutputMaster0_2_NoInput = float4(0,0,0,0);
             float4 VertexOutputMaster0_3_NoInput = float4(0,0,0,0);
             }
             

             void surf (Input IN, inout EditorSurfaceOutput o) {
                 o.Normal = float3(0.0,0.0,1.0);
                 o.Alpha = 1.0;
                 o.Albedo = 0.0;
                 o.Emission = 0.0;
                 o.Gloss = 0.0;
                 o.Specular = 0.0;
                 o.Custom = 0.0;
                 
                 float4 Master0_0_NoInput = float4(0,0,0,0);
                 float4 Master0_1_NoInput = float4(0,0,1,1);
                 float4 Master0_2_NoInput = float4(0,0,0,0);
                 float4 Master0_3_NoInput = float4(0,0,0,0);
                 float4 Master0_4_NoInput = float4(0,0,0,0);
                 float4 Master0_5_NoInput = float4(1,1,1,1);
                 float4 Master0_7_NoInput = float4(0,0,0,0);
                 float4 Master0_6_NoInput = float4(1,1,1,1);

                 o.Normal = normalize(o.Normal);
             }
         ENDCG
     }
     Fallback "Diffuse"
 }

(Done with Strumpy's Shader Editor, so there might be some redundant code there, since I don't understand half of it)