Custom Item Textures (No Resource Pack)

So, in Minecraft we have custom blocks and custom items. All of this requires resource packs and personally I dont like making resource packs. Now for custom blocks we can used player head items displays to give blocks a custom texture. You simply get a custom player head and scale it up to cover a block. With some other commands you can make that work.

I wish we had something similar to this but with items. I don't know how it would exactly work, but perhaps it could be a part of the skin model/texture that is not displayed. Then it would be virtually the same system.

Thank you for your time.