On a trans-dermal power source
I really think a trans-dermal power source is viable. It would require a bone anchor and extensive surgery though.
Perhaps even replacing a small bone with an artificial one that incorporates both battery and charger. The issue of the skin pulling back has already been solved. http://imgur.com/DgRMuV0
I'm not sure where I got that image from, it was a article on trans-dermal prosthetics in dogs.
It would simply require transplanting of cultivating more of the perlosteum prior to implantation, to cover all of the that would remain inside. To help protect against infection, the portion of the implant that sticks out would need to be patterned with Sharklet, or some similar substance.
The skin around the implant that would be held to the perlosteum would also need to be reinforced to be more durable somehow.
Another issue I have seen people raise here is the issue of power cables. Yes, the higher amperage cables require very tight connections, which might be painful when being pressed down upon. But it should be relatively easy to create a cable that is held on with a small magnet instead of tension, and just holds the transfer surfaces together that way. One of the contact surfaces would just need to extend slightly further than the other, to insure they didn't get hooked up backwards.
The power coupling would need a water/dust proof cover, which will make the exterior slightly bulkier. All of the newer Olympus cameras have small, light-weight covers over their battery compartments, and a similar design would work admirably.
As to where such a thing would go, I think it would best work as a replacement for a clavicle. There is plenty of (relatively) empty space surrounding the clavicle, so that the dimensions of the implant could be varied a bit to fit functionality. I have zero experience with CAD or any other design program, so I'm not sure how much movement range you could lose by changing the shape of the implant, though.