The World Of Warcraft characters are typically one mesh, which means they don’t have to worry about your problem. I would simply disable the teeth on the character when you go invisible. Alternatively you will have to implement the effect somewhat differently.
There is an interesting hack that you could try to use, but I am not sure how well it would perform. You can use a post process effect with Custom Stencil to target the teeth and run a depth check to see if they are occluded by other assets. Based on that result you can decide how they will be rendered. This is however quite overkill unless you really, really need the teeth to be visible in stealth.
I would either disable the teeth or implement the effect differently. You could try a Fresnel effect which typically gives a cool outline that could be thought of as a stealthy feeling.
Best luck Sengami 