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/renderInstance/renderTerrainMgr.cpp

@@ -35,6 +35,7 @@
 #include "terrain/terrCell.h"
 #include "terrain/terrCellMaterial.h"
 #include "math/util/matrixSet.h"
+#include "materials/materialManager.h"
 
 bool RenderTerrainMgr::smRenderWireframe = false;
 
@@ -117,6 +118,10 @@ void RenderTerrainMgr::render( SceneRenderState *state )
    if ( mInstVector.empty() )
       return;
 
+   // Check if bin is disabled in advanced lighting.
+   if ( MATMGR->getPrePassEnabled() && mBasicOnly )
+      return;
+
    PROFILE_SCOPE( RenderTerrainMgr_Render );
 
    GFXTransformSaver saver;