Hard coded triangle in HLSL geometry shader doesn't show on screen

Question

In Unity I'm planning on using a geometry shader for processing points into quads and can't quite figure out why I'm not getting output from my geometry shader. I've edited it down to a minimum working example, as seen here:

Shader "foo/bar"
{
    SubShader{
        Cull Off
        Lighting Off
        ZWrite Off

        Pass{
            CGPROGRAM
                #pragma vertex vert
                #pragma fragment frag
                #pragma geometry geom

                struct appdata {
                    float4 vertex : POSITION;
                };
                struct v2g {
                    float4 vertex : POSITION;
                };
                struct g2f {
                    float4 vertex : POSITION;
                };

                v2g vert(appdata v) {
                    v2g o = (v2g)0;
                    o.vertex = v.vertex;
                    return o;
                }

                [maxvertexcount(4)]
                void geom(point v2g p[1], inout TriangleStream<g2f> tristream)
                {
                    g2f o = (g2f)0;
                    o.vertex = float4(0.1, 0.1, 0, 0);
                    tristream.Append(o);
                    o.vertex = float4(0.1, 0.9, 0, 0);
                    tristream.Append(o);
                    o.vertex = float4(0.9, 0.9, 0, 0);
                    tristream.Append(o);
                }


                fixed4 frag(g2f i) : COLOR
                {
                    return fixed4(1,1,1,1);
                }
            ENDCG
        }
    }
}

I'd expect that to draw a white triangle covering just under half the texture I'm rendering to. Instead it displays all black, just as it was before the shader.

So far I've:

• Removed every possible source of culling I can think of
• Made absolute sure my setup works when rendering a mesh in a similar fashion
• Checked that this i getting input and running
• and scoured the very limited amount of documentation available

I'm all outta ideas. If anyone has even a minor clue as to what I'm doing wrong please let me know. Thanks.

-Fraser