How can I make rim lighting appear in front of everything?

I’m experimenting with rim lighting using a shader from the Unity Surface Shader Examples page. I would like to be able to have the rim light appear in front of all other objects so that it works as a highlighting system like in the pic below:

I would like to add a separate pass to the below shader where I only want to have the rim lighting. This way I can turn the ZWrite “On” and the ZTest to “Always” only for the rim lighting, and have it in front of everything but not the entire character. To ensure I don’t increase the drawcalls with the extra pass I’m going to have 2 versions of the shader; one with the extra rim lighting pass and one without, and switch between the two as necessary via script. I’ve looked at the Outline shader on the Unity Wiki but it’s process is a little too performance heavy so I’m trying to simplify things via rim lighting and avoid the complex calculations used in that one. Here is the shader code I’m using:

Shader "Example/Rim" {
    Properties {
      _MainTex ("Texture", 2D) = "white" {}
      _BumpMap ("Bumpmap", 2D) = "bump" {}
      _RimColor ("Rim Color", Color) = (0.26,0.19,0.16,0.0)
      _RimPower ("Rim Power", Range(0.5,8.0)) = 3.0
    SubShader {
      Tags { "RenderType" = "Opaque" }
      #pragma surface surf Lambert
      struct Input {
          float2 uv_MainTex;
          float2 uv_BumpMap;
          float3 viewDir;
      sampler2D _MainTex;
      sampler2D _BumpMap;
      float4 _RimColor;
      float _RimPower;
      void surf (Input IN, inout SurfaceOutput o) {
          o.Albedo = tex2D (_MainTex, IN.uv_MainTex).rgb;
          o.Normal = UnpackNormal (tex2D (_BumpMap, IN.uv_BumpMap));
          half rim = 1.0 - saturate(dot (normalize(IN.viewDir), o.Normal));
          o.Emission = _RimColor.rgb * pow (rim, _RimPower);
    Fallback "Diffuse"

How can I add an extra pass for the rim lighting?

You can’t separate the logic of surface shaders into separate passes; you can think of surface shaders as a bit like shader “wizards” - they provide a template and simplify the coding required to create many common shader effects, but at the expense of fine control over how the resulting code is structured.

If you want to create multiple pass shaders you’ll have to learn vertex/fragment coding: Unity - Manual: HLSL in Unity