Separate color/alpha blending problem

I am trying to create a shader which alpha blends a solid color over some rendered geometry in a second pass. I do this to highlight the object in the world as can be seen in games like Return To Castle Wolfenstein. I plan to do this by storing the blend factor in the src alpha channel in the second pass and use this as a blend factor when blending with the geometry rendered in the first pass. My shader code is as follows:

Shader "Custom/Color" { Properties { _Color ("Main Color", Color) = (1,0.5,0.5,1) _MainTex("Base (RGB)", 2D) = "white" {} _Cutoff("Alpha cutoff", Range(0, 1)) = 0.5 } SubShader { Tags { "Queue" = "AlphaTest" "RenderType" = "TransparentCutout" }

     //Cull front

     CGPROGRAM
     #pragma surface surf Ramp alphatest:_Cutoff vertex:vert
         float4 _Color;

     sampler2D _MainTex;

     // Set by script:
     // Shader.SetGlobalTexture("_Ramp", MY_TOON_RAMP)
     sampler2D _Ramp;

     struct Input
     {
         float2 uv_MainTex;
         float3 worldNormal;
     };

     void vert(inout appdata_full v, out Input o)
     {
         //v.normal = v.normal * -1;
     }

     half4 LightingRamp(SurfaceOutput s, half3 lightDir, half atten)
     {
         half NdotL = dot(s.Normal, lightDir);
         half diff =  NdotL * 0.5 + 0.5;
         half3 ramp = tex2D(_Ramp, float2(diff)).rgb;
         half4 c;
         c.rgb = s.Albedo * _LightColor0.rgb * ramp * (atten * 2);
         c.a = s.Alpha;
         return c;
     }

     void surf (Input IN, inout SurfaceOutput o)
     {
         half4 c = tex2D(_MainTex, IN.uv_MainTex);
         o.Albedo = c.rgb;
         o.Alpha = c.a;
     }
     ENDCG
     
     //Cull front
     Blend SrcAlpha OneMinusSrcAlpha, Zero One
     Pass{
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
             #include "UnityCG.cginc"
             float4 _Color;
             sampler2D _MainTex;
             float _factor;
             struct v2f {
                float4 pos : SV_POSITION;
                float3 color : COLOR0;
                float2 uv : TEXCOORD0;
             };
             
             float4 _MainTex_ST;
             
             v2f vert (appdata_base v)
             {
                v2f o;
                o.pos = mul (UNITY_MATRIX_MVP, v.vertex);
                o.color = _Color;
                o.uv = TRANSFORM_TEX(v.texcoord, _MainTex);
                return o;
             }
             
             half4 frag (v2f i) : COLOR
             {
                 half4 texcol = tex2D(_MainTex, i.uv);
                //return (texcol*2) * _Color;
                float4 color = _Color;
                color.a = texcol.a;
                return color;
             }
             ENDCG
     }
 } 
 FallBack "Diffuse"

}

The important line is:

Blend SrcAlpha OneMinusSrcAlpha, Zero One. Here I intend to use the alpha channel of the prerendered object which has the cookie cutter code so I still get the cookie cutter effect (via the seperate alpha channel blend Zero One) but this has no effect. The separate alpha channel blend is ignored. I would expect that if I set this to Zero Zero nothing would be rendered but it is still rendered and the alpha channel blend is performed using the color channel blend factors. How do I perform separate alpha channel blending?