Atangeo Balancer, a highly efficient polygon reduction tool has recently updated to version 1.2 which now boasts selective density controls as a way to better control the mesh density for different parts of the same model. With Atangeo Balancer 1.2, you can now control face densities for selective reduction rate of different groups, meshes, or sub-meshes, so in practice, you can set density to 5 for the head group of your model to increase the density of the head vertices roughly 5 times. Check out the details of selective density controls here:  http://www.atangeo.com/guide/density