Getting projector shader to conform to UVs

I've been trying to figure out how to generate an effect where a constantly increasing region of a level, centered on the player, changes to a different texture. I eventually concluded that using a projector would be a good way of getting that effect. I have very, very little experience with shaders, and after several hours of taking a built-in shader and hacking away at it, I managed to get something that almost does exactly what I want:

 Shader "Projector/Test" {
       Properties {
             _Color ("Main Color", Color) = (1,1,1,1)       
          _ShadowTex ("Cookie", 2D) = "" { TexGen ObjectLinear }
          _FalloffTex ("FallOff", 2D) = "" { TexGen ObjectLinear }
          _Mask ("Mask", 2D) = "" { TexGen ObjectLinear }
       }
       Subshader {
          Pass {
             ZWrite off
             Fog { Color (0, 0, 0) }
             Color [_Color]
             ColorMask RGB
             Blend SrcAlpha OneMinusSrcAlpha
             Offset -1, -1
             SetTexture [_Mask] {
                 combine texture * primary, ONE - texture
                 Matrix [_Projector]
             }
             SetTexture [_ShadowTex] {
                 constantColor (0,0,0,0)
                 combine texture lerp (previous) constant
             }
             SetTexture [_FalloffTex] {
                constantColor (0,0,0,0)
                combine previous lerp (texture) constant
                Matrix [_ProjectorClip]
             }
          }
       }
     }

The problem is that this doesn't take UVs into account; it simply plasters a texture onto the side of a level. Does anyone here know any good way to make it so the applied texture conforms to UVs? I'm sorry if I'm not being descriptive enough; I'm really out of my element here. Thanks in advance.