Hmmnnnn, okay, from what I'm seeing, and, (this is assuming a flection body is hard plastic), if you're able to warm the plastic up and then pull the neck mechanism out (you'd pop the mechanism back in the same way, assuming this works in the first place), you should be able to dremel the neck down to a size you want. I'm staring as hard as I can at all the necks and they seem to be uniform in width (make sure the inside looks uniform as well before attempting it). I would also take note of the way your doll's neckline is cut so you can imitate it after you've dremeled to a size you like. (What I mean is..notice how on some of them in the front-on view have a "U" shape cut out of the front (and other ones have "U" shapes cut on the sides and back as well) this is so the neck joint can pivot and the doll's head can lean forward and look down or to the sides, etc.) It wouldn't be an easy fix, and it would require patience, but unless this mysterious ball joint is connected to something else (I'm betting it's probably not), you could, in theory (I say in theory because I haven't actually sat down and done this myself, but I'm thinking it would work
) dremel the neck down to a more acceptable size.
Just:
-Pay attention to the inside of the "tube" of the neck, if it widens at any point, don't go near that point, remember the ball joint is going to have to fit snugly back inside the neck when this is all done.
-Try and trace, or keep a photographic record of what your doll's neckline originally looked like so you can create it with the dremel.
-The dremel's going to leave you with a pretty rough neckline, so take a higher grit sandpaper to smooth it out the best you can.
Then again, that part is going to be hidden by a head anyway, so it's all up to your own aesthetics.
To pop the neck joint in and out, follow one of the many action figure guides out there (they call it the "boil'n'pop" method) or here's a simplified version:
-Boil a pot of water on the stove. I would let it juuuust start to boil (when you start to see little bubbles coming up the sides of the pot), and then take it off and let it sit for a minute. Take your doll and submerge just her neck, ideally try and find a way for her to balance over the pot with just her neck submerged for around 2-5 minutes (you can test it at two minutes and if it's still not hot I would try again for another three minutes). Take the doll out and have a dry rag ready to grip the peg head with. Pull firmly. You may have to exert a bit of pressure to pop the ball joint out of the neck (that's the whole idea), but if you find yourself sweating with exertion and the thing isn't budging, I'd personally abort the mission because ultimately your zest for the project is going to be stronger than the plastic plug and it will break, but I do want to make it clear this isn't going to 'slide out like buttah'.
If everything goes well, POP! Now you have a loose neck peg-and-ball mechanism and a warm doll neck. Wait until she cools down before you start your dremeling, you don't want to stress the plastic, which is somewhat fragile to warping at the moment because it's been heated.
Dremel down to your desired length and shape.
Repeat the process to put the peg back in, but you'll be heating the neck up and pushing the peg in instead of pulling it out. This should actually be easier because the peg is going to be back to it's full hardness while the neck plastic will again, through the boiling, be "soft" (relatively speaking) and a bit more pliable.
If the doll neck/body is vinyl...scratch this entire post.
I tried?