"Nevertheless, the problem remains that a high SubD value must be set for the entire mesh"

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Wouldn't that be awesome if a subdivision area could be controlled by a map or a morph?

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I recently tried a very crude "real world" solution to that problem:

I exported a subdivded torso and stitched the unsubdivided arms and head back on to create a "high res / low res hybrid mesh", as most detail "happens" in the torso area anyway. (Ribcage, shoulder blades, spine)

Unfortunately the UV-coordinates got messed up along the seams, so I abandoned the project.