Because, of course, I broke the first build...

fml/client/cpw/mods/fml/client/FMLClientHandler.java

@@ -630,6 +630,7 @@ public class FMLClientHandler implements IFMLSidedHandler
         info.override = overridingTexturePath;
         info.texture = textureToOverride;
         list.add(info);
+        FMLCommonHandler.instance().getFMLLogger().log(Level.FINE, String.format("Overriding %s @ %d with %s. %d slots remaining",textureToOverride, location, overridingTexturePath, SpriteHelper.freeSlotCount(textureToOverride)));
     }
     /**
      * @param mod
@@ -897,9 +898,10 @@ public class FMLClientHandler implements IFMLSidedHandler
     /**
      * 
      */
-    public void preGameLoad()
+    public void preGameLoad(String user, String sessionToken)
     {
-        // NOOP at the minute
+        // Currently this does nothing, but it's possible I could relaunch Minecraft in a new classloader if I wished
+        Minecraft.fmlReentry(user, sessionToken);
     }
 
     /**

fml/client/cpw/mods/fml/client/SpriteHelper.java

@@ -110,4 +110,13 @@ public class SpriteHelper
         return next;
     }
 
+    /**
+     * @param textureToOverride
+     * @return
+     */
+    public static int freeSlotCount(String textureToOverride)
+    {
+        return spriteInfo.get(textureToOverride).cardinality();
+    }
+
 }

fml/patches/minecraft/net/minecraft/client/Minecraft.java.patch

@@ -55,13 +55,14 @@
          this.field_6287_N = System.currentTimeMillis();
      }
  
-@@ -2250,7 +2260,10 @@
+@@ -2250,7 +2260,11 @@
          {
              var2 = p_main_0_[1];
          }
 -
-+        FMLClientHandler.instance().preGameLoad();
++        FMLClientHandler.instance().preGameLoad(var1, var2);
 +    }
++    
 +    public static void fmlReentry(String var1, String var2)
 +    {
          func_6269_a(var1, var2);