[Deferred Decals] Add simple specular channel to deffered decals

I'm trying to use Unity's deferred decals (the ones that Unity announced with the release of Unity 5). I want to add a simple specular channel to the shader. With surface shaders, that's fairly easy to do, but the decal shader is more complicated and I don't fully understand it. Here's what I've got (not much):

 Shader "Decal/DecalShader"
 {
     Properties
     {
         _MainTex ("Diffuse", 2D) = "white" {}
     }
     SubShader
     {
         Pass
         {
             Fog { Mode Off } // no fog in g-buffers pass
             ZWrite Off
             Blend SrcAlpha OneMinusSrcAlpha
 
             CGPROGRAM
             #pragma target 3.0
             #pragma vertex vert
             #pragma fragment frag
             #pragma exclude_renderers nomrt
             
             #include "UnityCG.cginc"
 
             struct v2f
             {
                 float4 pos : SV_POSITION;
                 half2 uv : TEXCOORD0;
                 float4 screenUV : TEXCOORD1;
                 float3 ray : TEXCOORD2;
                 half3 orientation : TEXCOORD3;
             };
 
             v2f vert (float3 v : POSITION)
             {
                 v2f o;
                 o.pos = mul (UNITY_MATRIX_MVP, float4(v,1));
                 o.uv = v.xz+0.5;
                 o.screenUV = ComputeScreenPos (o.pos);
                 o.ray = mul (UNITY_MATRIX_MV, float4(v,1)).xyz * float3(-1,-1,1);
                 o.orientation = mul ((float3x3)_Object2World, float3(0,1,0));
                 return o;
             }
 
             CBUFFER_START(UnityPerCamera2)
             float4x4 _CameraToWorld;
             CBUFFER_END
 
             sampler2D _MainTex;
             sampler2D_float _CameraDepthTexture;
             sampler2D _NormalsCopy;
     
             fixed4 frag(v2f i) : SV_Target
             {
                 i.ray = i.ray * (_ProjectionParams.z / i.ray.z);
                 float2 uv = i.screenUV.xy / i.screenUV.w;
                 // read depth and reconstruct world position
                 float depth = SAMPLE_DEPTH_TEXTURE(_CameraDepthTexture, uv);
                 depth = Linear01Depth (depth);
                 float4 vpos = float4(i.ray * depth,1);
                 float3 wpos = mul (_CameraToWorld, vpos).xyz;
                 float3 opos = mul (_World2Object, float4(wpos,1)).xyz;
 
                 clip (float3(0.5,0.5,0.5) - abs(opos.xyz));
 
 
                 i.uv = opos.xz+0.5;
 
                 half3 normal = tex2D(_NormalsCopy, uv).rgb;
                 fixed3 wnormal = normal.rgb * 2.0 - 1.0;
                 clip (dot(wnormal, i.orientation) - 0.3);
 
                 fixed4 col = tex2D (_MainTex, i.uv);
                 return col;
             }
             ENDCG
         }        
 
     }
 
     Fallback Off
 }