This is pretty easy to do, actually. Use the IItemRenderer interface to override item rendering. Here's an example:
package whatever;
import net.minecraft.src.*;
import net.minecraftforge.client.IItemRenderer;
public class TestRenderer implements IItemRenderer
{
public boolean handleRenderType(ItemStack item, ItemRenderType type)
{
if (type == ItemRenderType.EQUIPPED) { return true; }
return false;
}
public boolean shouldUseRenderHelper(ItemRenderType type, ItemStack item, ItemRendererHelper helper)
{
return false;
}
public void renderItem(ItemRenderType type, ItemStack item, Object... data)
{
if (type == ItemRenderType.EQUIPPED) {
RenderBlocks blockRenderer = (RenderBlocks)data[0];
EntityLiving entity = (EntityLiving)data[1]; // entity holding the item
// render item here
}
}
}
Register it like this in your client proxy:
MinecraftForgeClient.registerItemRenderer(ITEMID, new TestRenderer());