Quick implementation of the first step documented in http://prideout.net/blog/?p=51

ThickGlass.cginc

#include "UnityCG.cginc"

fixed4 _Color;
float _DepthScale;
float _Sigma;

struct VertexInput
{
    float3 objectPos    : POSITION;
};

struct FragmentInput
{
    float4 clipPos      : POSITION;
    float3 viewPos      : TEXCOORD0;
    float3 objectPos    : TEXCOORD1;
};

FragmentInput Vertex(VertexInput input)
{
    FragmentInput output;
    output.clipPos = UnityObjectToClipPos(input.objectPos);
    output.viewPos = UnityObjectToViewPos(input.objectPos);
    output.objectPos = input.objectPos;
    return output;
}

float4 Fragment(FragmentInput input, fixed facing : VFACE) : SV_Target
{
#if UNITY_VFACE_FLIPPED
    facing = -facing;
#endif

    //TODO: How do we get a sensible gradient?
    //float density = pow(0.5 - input.objectPos.y * 0.5, 2);

    // float density = 1.0;

    float density = 0.5 + input.objectPos.y * 0.5;
    density = exp(-_Sigma * density);


    float depth = _DepthScale * density * input.viewPos.z * sign(facing);
    return depth * _Color;
}

ThickGlass.shader

Shader "Unlit/TempShader"
{
    Properties
    {
        _Color ("Color", Color) = (1,1,1,1)
        _DepthScale ("Depth Scale", Range(0.001, 2.0)) = 1.0
        _Sigma ("Sigma", Range(0.1, 100.0)) = 30.0
    }

    SubShader
    {
        Tags
        {
            "RenderType" = "Transparent"
            "Queue" = "Transparent+1"
        }

        LOD 100
        Blend One One
        Cull Off
        ZWrite Off

        Pass
        {
            CGPROGRAM
            #pragma vertex Vertex
            #pragma fragment Fragment
            #pragma target 3.0 // for VFACE
            #include "TempShader.cginc"
            ENDCG
        }
    }
}