Here's the "GrabPassFail.shader" shader mentioned in the resolution notes:

Shader "Test/GrayScal" {
Properties{
}
Category{
Tags{ "Queue" = "Transparent-50" "IgnoreProjector" = "True" "RenderType" = "Opaque" "PreviewType" = "Plane"}

SubShader{

GrabPass
{
"_BackgroundTexture"
}

// This should render the plane, cube and sphere again using _BackgroundTexture
Pass {
CGPROGRAM
#pragma vertex vert
#pragma fragment frag

#include "UnityCG.cginc"

#if STEREO_INSTANCING_ON
UNITY_DECLARE_TEX2DARRAY(_BackgroundTexture);
#else
UNITY_DECLARE_TEX2D(_BackgroundTexture);
#endif

struct appdata_t {
float4 vertex : POSITION;
UNITY_VERTEX_INPUT_INSTANCE_ID
};

struct v2f {
float4 vertex : SV_POSITION;
float4 grabPos : TEXCOORD2;
UNITY_VERTEX_OUTPUT_STEREO
};

v2f vert(appdata_t v)
{
v2f o;
UNITY_SETUP_INSTANCE_ID(v);
UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(o);
o.vertex = UnityObjectToClipPos(v.vertex);
o.grabPos = ComputeGrabScreenPos(o.vertex);

return o;
}

float4 frag(v2f i) : SV_Target
{
UNITY_SETUP_STEREO_EYE_INDEX_POST_VERTEX(i);
float2 proj = i.grabPos.xy / i.grabPos.w;

#if STEREO_INSTANCING_ON
float3 proj3 = float3(proj, unity_StereoEyeIndex);
half4 bgcolor = UNITY_SAMPLE_TEX2DARRAY(_BackgroundTexture, proj3);
#else
half4 bgcolor = UNITY_SAMPLE_TEX2D(_BackgroundTexture, proj);
#endif

float grayValue = dot(bgcolor.rgb, float3(0.3, 0.59, 0.11));
return float4(grayValue, grayValue, grayValue, 1);
}
ENDCG
}
}
}
}