For anyone interested, I managed to find a solution to this by combining two shaders from the forums. Here is the working shader:

Shader "FX/Matte Shadow Mask" {

Properties {
 _Color ("Main Color", Color) = (1,1,1,1)
 _MainTex ("Base (RGB) Trans (A)", 2D) = "white" {}
 _Cutoff ("Alpha cutoff", Range(0,1)) = 0.5

}


SubShader {

 Tags {"Queue" = "Geometry-10" "IgnoreProjector"="True" "RenderType"="TransparentCutout"}
 LOD 200
 Blend Zero SrcColor
 Lighting Off
 ZTest LEqual
 ZWrite On
 ColorMask 0
 Pass {}

CGPROGRAM
#pragma surface surf ShadowOnly alphatest:_Cutoff


fixed4 _Color;

struct Input {

 float2 uv_MainTex;

};

inline fixed4 LightingShadowOnly (SurfaceOutput s, fixed3 lightDir, fixed atten)

{
 fixed4 c;
 c.rgb = s.Albedo*atten;
 c.a = s.Alpha;
 return c;
}

void surf (Input IN, inout SurfaceOutput o) {
 fixed4 c = _Color;
 o.Albedo = c.rgb;
 o.Alpha = 1;
}
ENDCG
}
Fallback "Transparent/Cutout/VertexLit"
}