If I were to do the hollowing-out of a TPE doll to make it lighter, then for hard areas (ribcage, maybe thighs) I'd do the filling with spray-foam-in-a-can.
First I would hollow out the areas as needed, then I would insert plastic bags (that I'd previously tested for TPE compatibility by extended contact, etc., with some of the removed TPE) into the areas to be filled with foam. Then I'd put the foam-can's dispenser onto a straw that is inserted into the first bag, and fill that with enough foam that when it finishes expanding, it would fill that completely, but not push excessively hard on the surrounding TPE. I'd leave the straw there and remove the can's dispenser, so excess foam can push back out the straw for a while (with coverings around that so it doesn't get on anything; it's REALLY hard to get off, basically acting like foamed superglue).
Once that area was filled and cured, I'd go on to the next hollowed area. (only one area at a time means that if something "goes wrong" there's only one to clean up at a time, etc).
Alternately, a "mold" could be made to put the bag inside of, and then fill that, then insert the cured bagged foam core into the hollowed-out TPE...but this means being much more precise in the TPE removal to hollow out the space, as it has to exactly match the molded item, or you end up with a lumpy/distorted TPE outside of that.
So it's simpler to inject the foam into the hollowed TPE directly (inside a bag).
Since the bagged foam presumably fills the entire area around the skeleton, it should be pretty stiffly "attached" to it, if the bag goes around the metal skeleton (meaning, folds around it--you could actually bag up the skeleton itself so the foam adheres to the metal, but it may be difficult to tape the bag to the skeleton where the metal tubes would exit the bag, tightly enough to prevent foam from leaking out of the bag while expanding.
If something really doesn't work out about it, the foam can be removed relatively easily, since none of it is stuck to the TPE, and you can just slit the TPE open just enough to remove the core, then close it back up and fill it with something else instead. Or break up the foam and pull it out thru a smaller hole.
There are different densities of sprayfoam, so you can make a really hard core, or a softer one that will crush if really pressed on (but this is a permanent crushing with this type of sprayfoam).
For the "guts" I'd probably stuff the hollowed area with dense cotton batting, or some medium-density squishy foam (like seat cushions). If I used the squishy foam, I'd cut the foam in shapes that fit in the hollowed spaces as closely as possible. If there isn't a way to do that, I'd use small chunks of it stuffed inside the hollowed space to fill it up. If I wanted a "realistic"(ish) feel, I'd use "tubes" of squishy foam, basically cut like sausages, to feel a little like intestines, and other shapes like kidneys, etc.
Myself, I wouldn't go that far...but some people might want to.
There *are* sprayfoams that don't harden that much, or even stay pretty soft, but they're not something you find at Lowes or HomeDepot like the other stuff, and may require more hazmat protections. (and probably be more expensive)
