You could create a new tissue within the creature's raw that could act like a second skin. For example, here is a skin-like tissue, using the same values as regular skin in-game:
[TISSUE:SECOND_SKIN]
[TISSUE_NAME:skin:NP]
[SCARS]
[TISSUE_MATERIAL:LOCAL_CREATURE_MAT:SKIN]
[RELATIVE_THICKNESS:1]
[CONNECTS]
[VASCULAR:1]
[PAIN_RECEPTORS:5]
[HEALING_RATE:100]
[TISSUE_SHAPE:LAYER]
Once you put that into a creature's raws, you can apply it to only the torso using
[TISSUE_LAYER:BY_CATEGORY:BODY:SECOND_SKIN]
It may need adjusting, since BY_CATEGORY:BODY will only work for creatures that have one body part that acts as both their upper and lower bodies. If you were making it for creatures with separate upper and lower bodies then you would need two [TISSUE_LAYER] tokens:
[TISSUE_LAYER:BY_CATEGORY:BODY_UPPER:SECOND_SKIN]
[TISSUE_LAYER:BY_CATEGORY:BODY_LOWER:SECOND_SKIN]
Though this tissue will only be usable for the creature that has it in it's raw file. To create a new, 'true' second skin you would have to go into b_detail_plan_default.txt, go to [BODY_DETAIL_PLAN:VERTEBRATE_TISSUE_LAYERS] and add a new argument value to the end of each body part you want to be covered by the second skin. The argument should be called arg6, since there are already 5 potential arguments.
Now when you define a creature's tissues using [BODY_DETAIL_PLAN:VERTEBRATE_TISSUE_LAYERS:SKIN:FAT:MUSCLE:BONE:CARTILAGE] you can add the 6th "skin covering" tissue after cartilage and it should be applied over the body parts that have the value arg6.
I haven't actually done this before, so if anyone could point out any mistakes I made I would appreciate it.