How do I add alpha writes to this shader?

I draw the camera output to a RenderTexture with sprites drawn with the following shader (2D normal mapped sprite). The sprites appear when drawn to the screen, but are invisible when drawn to the RenderTexture. What do I need to change to write opacity to the RenderTexture? Thanks!

Code (CSharp):

1. Shader "SG/_ShipSurfaceShader" {
2.     Properties {
3.         _MainTex ("Main texture", 2D) = "white" {}
4.         _NormalTex ("Normal map", 2D) = "bump" {}
5.         _SpecColor ("Specular colour", Color) = (1,1,1,1)
6.         _SpecPower ("Specular power", Range (0.0,2.0)) = 0.5
7.     }
8.     SubShader {
9.         Tags { "Queue"="Transparent" }
10.         Blend SrcAlpha OneMinusSrcAlpha
11.         LOD 200
12.        
13.         CGPROGRAM
14.         #pragma surface surf BlinnPhongTransparent alpha
15.  
16.         inline fixed4 LightingBlinnPhongTransparent (SurfaceOutput s, fixed3 lightDir, half3 viewDir, fixed atten) {
17.             half3 h = normalize (lightDir + viewDir);
18.             fixed diff = max (0, dot (s.Normal, lightDir));
19.             float nh = max (0, dot (s.Normal, h));
20.             float spec = pow (nh, s.Specular*32.0) * s.Gloss;
21.             fixed4 c;
22.             c.rgb = (s.Albedo * _LightColor0.rgb * diff + _LightColor0.rgb * _SpecColor.rgb * spec) * (atten);
23.             c.a = s.Alpha;
24.             return c;
25.         }
26.  
27.         sampler2D _MainTex;
28.         sampler2D _NormalTex;
29.         half _SpecPower;
30.  
31.         struct Input {
32.             float2 uv_MainTex;
33.         };
34.  
35.         void surf (Input IN, inout SurfaceOutput o) {
36.             fixed4 c = tex2D (_MainTex, IN.uv_MainTex);
37.             o.Albedo = c.rgb;
38.             o.Alpha = c.a;
39.             o.Normal = UnpackNormal(tex2D (_NormalTex, IN.uv_MainTex));
40.             o.Specular = _SpecPower;
41.             o.Gloss = 1;
42.         }
43.         ENDCG
44.     }
45.     FallBack "Diffuse"
46. }
47.  

The below shows a quad with the RenderTexture applied visible in the scene. The planet and waypoint (Unity Sprites) are visible but the ship using the above shader isn't.