Rendering ghost items

I want to be able to render a "ghost" item - essentially just a semi-transparent item's icon - but I can't figure out how to do it.

Searched online, of course. Just got a bunch of bug fix reports about a different type of ghost item. Tried to figure out how vanilla does it, and eventually I jerry-rigged this non-functional thingamajig.

    @Override
    protected void drawGuiContainerBackgroundLayer(float opacity, int x, int y) {
    ...
        // Render ghost item
        if (tileEntity.getMatchingRecipe() != null) {
            ItemStack item = tileEntity.getMatchingRecipe().getResult();
            IItemRenderer customRenderer = MinecraftForgeClient
                    .getItemRenderer(item, INVENTORY);
            customRenderer.renderItem(INVENTORY, item);
        }
    }

I get a NPE on customRenderer.renderItem(ItemRenderType, ItemStack). I have no idea what I thought would happen, I just plugged it in to at least try something.

So, anybody have any experience with this?