Depth mask with alpha

Hello,
I'm trying to achieve a fade out of multiple objects by using a plane in front of them that renders the skybox.
The cutout to skybox is the easy part – just making the shader render before the geometry (geometry-10) works perfectly.
The problem is adding alpha to the rendered pixels during the first pass, so to achieve the fade. If I try to add another pass, it has no knowledge of the currently rendered pixels and adding alpha will just fade to black.
I'm having problems in understanding how to make a grab pass here because all examples and tuts I can find blend with textures, which is really not what I want here.
Does anybody have a good idea on how to do this or some good resources I can read about this?

My shader currently looks like this.

Code (CSharp):

1. Shader "Custom/NewMask"
2. {
3.    Properties
4.     {
5.         _Color ("Color", Color) = (1,1,1,1)
6.  
7.     }
8.     SubShader
9.     {
10.         Tags {"Queue" = "Geometry-10" "RenderType"="Transparent" }
11.            ZWrite On
12.         AlphaToMask On
13.         ColorMask A
14.  
15.         Pass {
16.         }
17.  
18.         CGPROGRAM
19.        
20.         #pragma surface surf Standard fullforwardshadows alpha:fade
21.         #pragma target 3.0
22.        
23.         sampler2D _MainTex;
24.        
25.  
26.         struct Input {
27.             float2 uv_MainTex;
28.         };
29.  
30.         fixed4 _Color;
31.         void surf (Input IN, inout SurfaceOutputStandard o)
32.         {
33.             fixed4 c = _Color;
34.             //o.Albedo = c.rgb;
35.  
36.             o.Alpha = c.a;
37.         }
38.         ENDCG
39.     }
40.     FallBack "Standard"
41. }
42.  

 

Bump!
The more I look around, the more I think this shader could be a neat solution to a problem: fading out multiple elements is quite a pain to do with normal shaders and complex geometries/hierarchies and sometimes you could just have a plane in front of the object that fades to the underlying scene. It could be useful in a number of (very controlled) situations.

I'm pretty sure that I'm missing out something on how to further elaborate a depth mask shader. No matter what I try, I'm back at the starting point – I'm only able to depth mask but not to apply alpha to the masking object.

Any suggestions?

 

Still struggling with this ) :
I've implemented the grabpass, renders above what I want to fade and I'm to edit the color of the fragments like I want so it's pretty much there, but when it comes to alpha, it just fades to black. Am I missing something? Does anybody know what I'm doing wrong?
Thanks!

Code (CSharp):

1. Shader "Custom/Maskv5"
2. {
3.     Properties
4.     { _Color ("Color", Color) = (0.5, 0.5, 0.5, 0.5) }
5.     SubShader
6.     {
7.  
8.         Tags { "Queue" = "Transparent-1"}
9.  
10.         // Grab the screen behind the object into _BackgroundTexture
11.         GrabPass
12.         {
13.             "_BackgroundTexture"
14.         }
15.         // Render the object with the texture generated above and apply alpha
16.  
17.         ZWrite On
18.         Pass
19.         {
20.             CGPROGRAM
21.             #pragma vertex vert
22.             #pragma fragment frag alpha
23.  
24.             #include "UnityCG.cginc"
25.             fixed4 _Color;
26.             struct v2f
27.             {
28.                 float4 grabPos : TEXCOORD0;
29.                 float4 pos : SV_POSITION;
30.             };
31.  
32.             v2f vert(appdata_base v) {
33.                 v2f o;
34.                 // use UnityObjectToClipPos from UnityCG.cginc to calculate
35.                 // the clip-space of the vertex
36.                 o.pos = UnityObjectToClipPos(v.vertex);
37.                 // use ComputeGrabScreenPos function from UnityCG.cginc
38.                 // to get the correct texture coordinate
39.                 o.grabPos = ComputeGrabScreenPos(o.pos);
40.                 return o;
41.             }
42.  
43.             sampler2D _BackgroundTexture;
44.             half4 frag(v2f i) : SV_Target
45.             {
46.  
47.                 half4 bgcolor = tex2Dproj(_BackgroundTexture, i.grabPos)-(0,0,0,_Color.a);
48.                 return bgcolor;
49.             }
50.             ENDCG
51.         }
52.  
53.     }
54. }