Add a new “harvest” butchering option. This is for when you are butchering and just trying to aquire materials not edible food. This would make sense for things like zombies. Your survivor probably wouldn’t focus on the meat of zombies as it is useless to them and instead focus exclusively on cutting out the fat and bones. The same goes for mutants. I could see a survivor mass mudering mutants purely to cut them up for their fat, bones/chitin and skin. This could save you a lot of time when you are trying to cut up and turn zombies into tallow before the fat rots away.
Maybe there should even be options for the specific part you are trying to harvest. Just like we have skinning to specifically harvest the skin of a creature, maybe we could have something similar for fat and bones. Harvesting specifically bones or chitin could be done very fast as these are hard things so you could simply cut/chop of the softer bits without having to be carefull or precise. Harvesting fat would be a bit slower but would still be useful when you are just trying to turn corpses into fuel.
I’ve done a bit of butchering in my day and a lot of dissections/skinning. Harvesting fat without harvesting meat isn’t going to be much of a time difference as they’re basically connected all throughout the body, and to get the bones you have to separate the ligaments and other attachment points from the skeletal musculature, so again you’re not going to actually save time by focusing on it.
As for hide/chitin there’s already a “skin” option to just harvest hide or chitin as you already mentioned in your post.
Also couldn’t you save time if you only went after bones or chitin becease you can forego a lot of precision and caution that you would normaly have butchering a animal. You don’t really have to worry about damaging the bone or chitin as much with your blade given how hard it is and the meat can just be roughly and quickly chopped/crapped of the bone while you don’t even bother separating the fat, muscle and organs. Maybe save even more time if you only went after large bones and pieces of chitin and just left the smaller ones?
Might it also be possible to havest fat quicker if you work less precise and ignore smaller bits. So you would have a tradeoff between saving time but having less harvest and fat that is so contaminated that you can’t make anything edible out of it but for things like biodiesel it should be fine.
I could have sworn I remember skinning giving chitin before, odd. As for the bones I could see the argument being made that just slicing in and carving out the bones could be quicker.
As for fat you could just carve out subcutaneous fat and ignore the skeletal muscle fat, but that would also basically require you to skin the creature anyways if you want all of it. You could focus on the major deposits, the weirdness there comes in that different animals carry their fat differently, but for a human/zombie you could probably get 80% of the total fat just focusing on the major areas.