Make a standard Unity Quad full-screen, purely in the shader

FullScreenQuad.shader

//
// 1. Add a Quad in Unity
// 2. Parent the quad under camera, to prevent frustum culling.
// 3. Attach this shader.
//
Shader "Quad/Fullscreen"
{
	Properties
	{
	}

	SubShader
	{
		Cull Off ZWrite Off

		Pass
		{
			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag

			#include "UnityCG.cginc"

			struct appdata
			{
				float4 vertex : POSITION;
				float2 uv     : TEXCOORD0;
			};

			struct v2f
			{
				float2 uv : TEXCOORD0;
				float4 vertex : SV_POSITION;
			};

			v2f vert(appdata v)
			{
				v2f o;
				o.vertex = v.vertex;
				o.vertex.xy *= 2;
				o.uv = v.uv;
				return o;
			}

			fixed4 frag(v2f i) : SV_Target
			{
				return fixed4(i.uv, 0, 1);
			}
			ENDCG
		}
	}
}

This version relies on the Unity implementation of the Quad verts, which could in theory change over time.

For an independent version, check out the first revision of this Gist.

Hi I'm trying to use this shader but having trouble merging this with the shader I already have. This is my shader:
`Shader "alphaMaskShader"
{
Properties{
_MainTex("Base (RGB)", 2D) = "white" {}
_Alpha("Alpha (A)", 2D) = "white" {}
}
SubShader{
Tags{ "RenderType" = "Transparent" "Queue" = "Overlay" }

	ZWrite Off
	ZTest Off

	Blend SrcAlpha OneMinusSrcAlpha
	//ColorMask RGB

	Pass{
	    SetTexture[_MainTex]{
	        Combine texture
            }
	    SetTexture[_Alpha]{
	        Combine previous, texture
            }
        }
}

}`

I tried inserting your gist right after the pass but it isn't rendering anything. Are these just incompatible?