Hello there! I was the one who created that thread at the modthesims forums. Anyway, you're going off the wrong direction here, bones are used for animation, and mostly animation, making certain sim body parts change more nicely, is done through the magical and mysterious thing as Dmaps!
Dmaps don't use bones for deformation and work by increasing or decreasing the position of a vertex. There is no butt bone for instance. Anyway, I coincidentally posted in the Dmap forum post here yesterday. Here's the forum post, by the way.
forums.thesims.com/en_US/discussion/797774/simgurumodsquad-0xdb43e069-deformermap-resource/p1We only know that much, we can't actually tell how the morphs are deforming the sim for the changes to the body parts. Likewise, we don't know how to change them without guessing. (Well, at least one person in the modding community knows, I've reached out to them but they haven't answered.)
Although the SimGurus won't be heading back to work for a while, it would be nice for you to go ahead and comment on the post so it can get more attention.