this is very computationally expensive to do such operation (to understand that teeth is completely occluded by something else like mouth)

this is called occlusion culling and usually it brings a lot of problems on its own (it's CPU intensive and you need to apply it smartly only where it helps)

and even if this occlusion culling would be cheap, the way how it is usually done requires precomputing a lot of data - so occluders are static geometry

since character head is skinned mesh - so dynamically changes every frame based on joints positions - that will be another level of complexity

game engines are not really helpful here