Untie ItemModelMesher from using Item Ids internally by implementing our own simple mechanics using Trove.

patches/minecraft/net/minecraft/client/renderer/entity/RenderItem.java.patch

@@ -1,5 +1,14 @@
 --- ../src-base/minecraft/net/minecraft/client/renderer/entity/RenderItem.java
 +++ ../src-work/minecraft/net/minecraft/client/renderer/entity/RenderItem.java
+@@ -83,7 +83,7 @@
+     public RenderItem(TextureManager p_i46165_1_, ModelManager p_i46165_2_)
+     {
+         this.field_175057_n = p_i46165_1_;
+-        this.field_175059_m = new ItemModelMesher(p_i46165_2_);
++        this.field_175059_m = new net.minecraftforge.client.ItemModelMesherForge(p_i46165_2_);
+         this.func_175041_b();
+     }
+ 
 @@ -302,6 +302,10 @@
                      modelresourcelocation = new ModelResourceLocation("bow_pulling_0", "inventory");
                  }

src/main/java/net/minecraftforge/client/ItemModelMesherForge.java

@@ -0,0 +1,82 @@
+package net.minecraftforge.client;
+
+import java.util.IdentityHashMap;
+import java.util.Iterator;
+import java.util.Map;
+import java.util.Map.Entry;
+
+import com.google.common.collect.Maps;
+
+import gnu.trove.map.hash.TIntObjectHashMap;
+import gnu.trove.procedure.TIntObjectProcedure;
+import net.minecraft.client.renderer.ItemModelMesher;
+import net.minecraft.client.resources.model.IBakedModel;
+import net.minecraft.client.resources.model.ModelManager;
+import net.minecraft.client.resources.model.ModelResourceLocation;
+import net.minecraft.item.Item;
+
+/**
+ * Wrapper around ItemModeMesher that cleans up the internal maps to respect ID remapping.
+ */
+public class ItemModelMesherForge extends ItemModelMesher
+{
+    IdentityHashMap<Item, TIntObjectHashMap<ModelResourceLocation>> locations = Maps.newIdentityHashMap();
+    IdentityHashMap<Item, TIntObjectHashMap<IBakedModel>> models = Maps.newIdentityHashMap();
+
+    public ItemModelMesherForge(ModelManager manager)
+    {
+        super(manager);
+    }
+
+    protected IBakedModel getItemModel(Item item, int meta)
+    {
+        TIntObjectHashMap<IBakedModel> map = models.get(item);
+        return map == null ? null : map.get(meta);
+    }
+
+    public void register(Item item, int meta, ModelResourceLocation location)
+    {
+        TIntObjectHashMap<ModelResourceLocation> locs = locations.get(item);
+        TIntObjectHashMap<IBakedModel>           mods = models.get(item);
+        if (locs == null)
+        {
+            locs = new TIntObjectHashMap();
+            locations.put(item, locs);
+        }
+        if (mods == null)
+        {
+            mods = new TIntObjectHashMap();
+            models.put(item, mods);
+        }
+        locs.put(meta, location);
+        mods.put(meta, getModelManager().getModel(location));
+    }
+
+    public void rebuildCache()
+    {
+        final ModelManager manager = this.getModelManager();
+        for (Map.Entry<Item, TIntObjectHashMap<ModelResourceLocation>> e : locations.entrySet())
+        {
+            TIntObjectHashMap<IBakedModel> mods = models.get(e.getKey());
+            if (mods != null)
+            {
+                mods.clear();
+            }
+            else
+            {
+                mods = new TIntObjectHashMap();
+                models.put(e.getKey(), mods);
+            }
+            final TIntObjectHashMap<IBakedModel> map = mods;
+            e.getValue().forEachEntry(new TIntObjectProcedure<ModelResourceLocation>()
+            {
+                @Override
+                public boolean execute(int meta, ModelResourceLocation location)
+                {
+                    map.put(meta, manager.getModel(location));
+                    return true;
+                }
+            });
+        }
+    }
+}