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Sadly, nothing offical has ever been addressed.

The closest thing available is the table in Rifter 3 (i.e.ptional), but that is converting the damages to MD.

That's one of the few drawbacks of Supernatural damage, since the damage is magical in nature, rather than based on real force, it doesn't like stacking with mundane physical advantages. Though a few supernatural creatures do have more damage with some physical attacks than they do with others, so there is precedent for supernatural creatures to have more damage focused on more threatening limbs.
..Another consideration could be allowing normal kick damage and supernatural damage to stack as per the optional stacking rules for weapons and SN. Which is to say that they better have a strong kick or a low supernatural strength to avoid overdoing it, or a tough leg.