eraser effect in plane using shader

i am trying to produce an eraser for a material on a plane. The way i am thinking of doing this is by passing an array to the shader telling the shader where the material should be transparent; if the value from the array is 0, i return no color for the material (i.e. tranparent). i have 2 problems:

1) how do i declare and pass an array in CG?

2) does this way work and if so, is this the best way to do this? I am thinking that it might be very demanding..

UPDATE

ok, so i have this shader:

 Shader "mShaders/Holes1" {
     Properties {
       _MainTex ("Texture (RGB)", 2D) = "white" {}
       _SliceGuide ("Slice Guide (RGB)", 2D) = "white" {}
       _SliceAmount ("Slice Amount", Range(0.0, 1.0)) = 0.9
     }
     SubShader {
       Tags { "RenderType" = "Opaque" }
       Cull Off
       CGPROGRAM
       //if you're not planning on using shadows, remove "addshadow" for better performance
       #pragma surface surf Lambert 
       //addshadow
       struct Input {
           float2 uv_MainTex;
           float2 uv_SliceGuide;
           float _SliceAmount;
       };
       sampler2D _MainTex;
       sampler2D _SliceGuide;
       float _SliceAmount;
       void surf (Input IN, inout SurfaceOutput o) {
           clip(tex2D (_SliceGuide, IN.uv_SliceGuide).rgb - _SliceAmount);
           o.Albedo = tex2D (_MainTex, IN.uv_MainTex).rgb;
       }
       ENDCG
     }
     Fallback "Diffuse"
   }

In my touch script i have the start function containing:

 texture = new Texture2D(32, 16);
     // set texture in the inspector slot
     renderer.material.SetTexture("_SliceGuide", texture);
     
     // Fill the texture with white (you could also paint it black, then draw with white)
     for (var y : int = 0; y < texture.height; ++y) 
     {
         for (var x : int = 0; x < texture.width; ++x) 
         {
             texture.SetPixel (x, y, Color.white);
         }
     }
     // Apply all SetPixel calls
     texture.Apply();

and in Update i have:

 if (Input.touchCount ==0  && !Input.anyKey) return;
     
     // Only if we hit something, do we continue
     var hit : RaycastHit;
     if (!Physics.Raycast (Camera.main.ScreenPointToRay(Input.mousePosition), hit)) return;
     
     // Just in case, also make sure the collider also has a renderer
     // material and texture. Also we should ignore primitive colliders.
     var renderer : Renderer = hit.collider.renderer;
     var meshCollider = hit.collider as MeshCollider;
     if (renderer == null || renderer.sharedMaterial == null ||texture == null || meshCollider == null) return;
     
     // Now draw a pixel where we hit the object
     var tex : Texture2D = texture;
     var pixelUV = hit.textureCoord;
     pixelUV.x *= tex.width;
     pixelUV.y *= tex.height;
     
     // add black spot, which is then transparent in the shader
     tex.SetPixel(pixelUV.x, pixelUV.y, Color.black);
     tex.Apply();

All seems to work on the computer, but on my iphone the object (here a mapped sphere) is rendered white with black spots after first touch (before first touch it's rendered ok with the texture on) -- the black spots are from where i touch the object. What am i doing wrong?