The final step (barring any overlooked missing bits, requested refactors, and of course, rolling in dependencies already submitted as PRs) consists of:

renderPrePassMgr.cpp related:
A) shifting .addFeature( MFT_XYZ); calls from ProcessedShaderMaterial::_determineFeatures to ProcessedPrePassMaterial::_determineFeatures
B) mimicking the "// set the XXX if different" entries from RenderMeshMgr::render in RenderPrePassMgr::render
C) fleshing out ProcessedPrePassMaterial::getNumStages() so that it shares a 1:1 correlation with ProcessedShaderMaterial::getNumStages()
D) causing inline void Swizzle<T, mapLength>::ToBuffer( void *destination, const void *source, const dsize_t size )  to silently fail rather than fatally assert if a source or destination buffer is not yet ready to be filled. (support for #customTarget scripted render targets)

Reflections:
A) removing reflectRenderState.disableAdvancedLightingBins(true); entries. this would otherwise early out from prepass and provide no color data whatsoever.
B) removing the fd.features.addFeature( MFT_ForwardShading ); entry forcing all materials to be forward lit when reflected.
C) 2 things best described bluntly as working hacks:
C1) when reflected, a scattersky is rotated PI along it's z then x axis in order to draw properly.
C2) along similar lines, in terraincellmaterial, we shut off culling if it's a prepass material.

Skies: scattersky is given a pair of rotations for reflection purposes, all sky objects are given a z value for depth testing.

Engine/source/materials/materialDefinition.cpp

@@ -162,6 +162,9 @@ Material::Material()
 
       mSeqFramePerSec[i] = 0.0f;
       mSeqSegSize[i] = 0.0f;
+
+      // Deferred Shading
+      mMatInfoFlags[i] = 0.0f;
    }
 
    dMemset(mCellIndex, 0, sizeof(mCellIndex));
@@ -170,6 +173,9 @@ Material::Material()
    dMemset(mNormalMapAtlas, 0, sizeof(mNormalMapAtlas));
    dMemset(mUseAnisotropic, 0, sizeof(mUseAnisotropic));
 
+   // Deferred Shading : Metalness
+   dMemset(mUseMetalness, 0, sizeof(mUseMetalness));
+
    mImposterLimits = Point4F::Zero;
 
    mDoubleSided = false;
@@ -204,6 +210,9 @@ Material::Material()
    
    mDirectSoundOcclusion = 1.f;
    mReverbSoundOcclusion = 1.0;
+
+   // Deferred Shading
+   mIsSky = false;
 }
 
 void Material::initPersistFields()
@@ -289,10 +298,7 @@ void Material::initPersistFields()
       
       addField( "useAnisotropic", TypeBool, Offset(mUseAnisotropic, Material), MAX_STAGES,
          "Use anisotropic filtering for the textures of this stage." );
-      
-      addField("envMap", TypeImageFilename, Offset(mEnvMapFilename, Material), MAX_STAGES,
-         "The name of an environment map cube map to apply to this material." );
-
+     
       addField("vertLit", TypeBool, Offset(mVertLit, Material), MAX_STAGES,
          "If true the vertex color is used for lighting." );
 
@@ -379,9 +385,6 @@ void Material::initPersistFields()
       addProtectedField("bumpTex",        TypeImageFilename,   Offset(mNormalMapFilename, Material),
          defaultProtectedSetNotEmptyFn, emptyStringProtectedGetFn, MAX_STAGES, 
          "For backwards compatibility.\n@see normalMap\n"); 
-      addProtectedField("envTex",         TypeImageFilename,   Offset(mEnvMapFilename, Material),
-         defaultProtectedSetNotEmptyFn, emptyStringProtectedGetFn, MAX_STAGES,
-         "For backwards compatibility.\n@see envMap\n"); 
       addProtectedField("colorMultiply",  TypeColorF,          Offset(mDiffuse, Material),
          defaultProtectedSetNotEmptyFn, emptyStringProtectedGetFn, MAX_STAGES,
          "For backwards compatibility.\n@see diffuseColor\n"); 
@@ -417,6 +420,9 @@ void Material::initPersistFields()
    addField("dynamicCubemap", TypeBool, Offset(mDynamicCubemap, Material),
       "Enables the material to use the dynamic cubemap from the ShapeBase object its applied to." );
 
+   addField("isSky", TypeBool, Offset(mIsSky, Material),
+       "Sky support. Alters draw dimensions." );
+
    addGroup( "Behavioral" );
 
       addField( "showFootprints", TypeBool, Offset( mShowFootprints, Material ),