Thank you for the replies.
Duplication and Summon seem problematic because the limb, unlike a hydra's heads, *wouldn't* act independently. It would still be controlled by the character and only act on his or her phases, etc. It's not a second complete character, or even an independently acting body part. It's just an arm or a leg, doing normal arm or leg things. While limitations and complications could model that, it seems much too complicated. And would Mind Link work as a stand-in for normal control over a limb, feeling, and natural coordination?
Honestly, I suspect Hugh Nielson is right that Duplication would work. It's just much, much more complicated than I'd prefer.
I think Grailknight's suggestion is similar to what I was thinking. If hit locations are used, there is a limit to the amount of BODY damage a limb can take. I was thinking that extra Body could be bought up to that amount (or thereabouts) with a partial coverage limitation. The character's normal BODY could have the (+0?) limitation "Only vs. damage to head and torso" (or "Not versus damage to right arm", etc.)
Foolishvictor makes a good point about resistant defenses. I can see how that would work fine for a cybernetic limb. Damage high enough to bypass the armor would be so catastrophic it does damage to the character's organic body as well.