It's nothing too difficult to do, all you need is a little imagination.
On eFlyFF what they did was they created a new targa image and then positioned it on the inventory window.
If you don't feel like it/know how to do it, you can just put a tab strip over the inventory texture in Photoshop.
In fact, allow me to spoonfeed you:
[Download]
All you need to do is replace the old WndInventory3D.tga in your client's theme with the one I've provided.
I haven't tested it but it SHOULD work. If it doesn't, you know what to do :)