Using VPOS

using_vpos.shader

// yeah it's not terribly nice

Shader "Foo" {
	SubShader {

		Pass {	
			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			#pragma target 3.0

			#include "UnityCG.cginc"

			#if defined(SHADER_API_D3D9)
			#define VPOS_TYPE float2
			#else
			#define VPOS_TYPE float4
			#endif

			struct appdata_t {
				float4 vertex : POSITION;
				float2 texcoord : TEXCOORD0;
			};

			struct v2f {
				float2 texcoord : TEXCOORD0;
			};

			v2f vert (appdata_t v, out float4 opos : SV_POSITION)
			{
				v2f o;
				opos = mul(UNITY_MATRIX_MVP, v.vertex);
				o.texcoord = v.texcoord;
				return o;
			}

			fixed4 frag (v2f i, VPOS_TYPE ppp : VPOS) : SV_Target
			{
				fixed4 c = 0;
				c.xy = ppp;
				c.xy *= 0.01;
				return c;
			}
			ENDCG 
		}
	}
}