Shader mask using greyscale instead of alpha?

I am completely new to writing shaders.

I found this simple shader which uses 2 textures, one main texture, and a second as a mask.

 Shader "MaskedTexture"
 {
    Properties
    {
       _MainTex ("Base (RGB) Trans (A)", 2D) = "white" {}
       _Mask ("Culling Mask", 2D) = "white" {}
       _Cutoff ("Alpha cutoff", Range (0,1)) = 0.1
    }
    SubShader
    {
       Tags {"Queue"="Transparent" }
       Lighting Off
       ZWrite Off
       Blend SrcAlpha OneMinusSrcAlpha
       AlphaTest GEqual [_Cutoff]
       Pass
       {
          SetTexture [_Mask] {combine texture}
          SetTexture [_MainTex] {combine texture, previous * texture}
       }
    }
 }

Source: http://www.clover-studio.com/blog/custom-mask-shader-for-unity-3d-weve-developed-for-a-childrenss-game/

This works great. However for my application I need to be able to do the exact same effect, except base the masking on greyscale values instead of alpha values.

Can this be easily modified to do that?