Yee i dont like the tweak table either... they dont look that good with most of the cosmetics.
Well this topic is kinda special for what i know... NPCs bodies can be changed because they arent complex files. The boss model however... its a combination of different type files, (morphs sliders, skin tones, hairs, eyes, tattoos, make up, colors). They can be edited yeah, but they have their limitations on what we can change and what we cant... But why? Well we never get the proper tools for this tasks, this is the same reason we cant add new hair styles or edit vehicles.
There are several mods for the players body (huge breasts, super muscle, etc) however, they are controlled by a xbtl file, so they wont be that different from the tweak table files.
Im not sure of what exactly you want to achieve lol. You can port a new body type and add it as a customization item with the workshop tools, but, it will only support body slider and colors customization , and it wont be able to use other clothes unless they are part of the new model.
