Rendering a Mesh with a CusomMaterial from a Custom RenderBinMgr

I have a custom RenderBinMgr-derived manager that processes tsMeshes that have a special flag set. I've got it processing in the correct order and handling the right meshes. Rendering the meshes in a custom way is the problem. I want each mesh it handles to use one particular CustomMaterial, but can't get that working. The shaders should transform the verts into screenspace, sample the backbuffer for the pixel color, and write the pixel depth to the depthbuffer. I just don't know enough about shaders and how T3D hooks into them to get this working. Here's my horrible attempt at accomplishing this...

myOccluderV.hlsl

struct VertData
{
   float3 position        : POSITION;
};

struct ConnectData
{
   float4 hpos            : POSITION;
};

uniform float4x4 modelview;

ConnectData main( VertData IN )
{
   ConnectData OUT;

   OUT.hpos = mul(worldview, float4(IN.position.xyz,1)).xzyw;
   
   return OUT;
}

myOccluderP.hlsl

uniform sampler2D backbuffer : register(S0);
// Where is the depthBuffer located???

struct ConnectData
{
   float4 hpos             : POSITION;
};

float4 main( ConnectData IN ) : COLOR   
{      
   float4 color = tex2D( backbuffer, IN.hpos.xy );
   
   float depth = IN.hpos.z / IN.hpos.w;
   
   // Need to write depth to the depthBuffer!
         
   return color;      
}

myOccluder.cs

singleton ShaderData( MyOccluderShader )
{
   DXVertexShaderFile   = "shaders/common/myOccluderV.hlsl";
   DXPixelShaderFile    = "shaders/common/myOccluderP.hlsl";

   pixVersion = 2.0;
};

//The render states used when rendering the post effect
singleton GFXStateBlockData( MyOccluderBlock )
{
   samplersDefined = true;
   samplerStates[0] = SamplerClampPoint;  // BackBuffer
//   cullDefined = true;
//   cullMode = "GFXCullCCW";
};

singleton CustomMaterial(myOccluder)
{
   version = 2;
   shader = MyOccluderShader;
   stateBlock = MyOccluderBlock;
   isOccluder = 1;
   sampler0 = "$backbuff";
   // Which sampler contains the depthbuffer?
};

For one thing, I don't know where the depthBuffer is stored. Second, the above code does not compile. T3D complains about "modelview" being undefined in the vertex shader. Also, do I need to do some setup in MyOccluderBinMgr::render() to pass the shaders any relevant data?