I made a thread with some arena mode testing but I can't find it again. In summary, off the top of my head plus some quick retesting:
* Toes, fingers, and facial features of the default humanoid form are not covered by armour.
* Toes and fingers contain fat, which melts and catches fire very easily and also bleeds a lot.
There are quite a lot of changes you can make to the raws to get around this.
* Give toes and fingers the [LIMB] token, allowing the armour stepping to reach them. This does allow dwarves to wrestle with individual fingers, and still leaves facial features exposed to general attacks.
* Remove fat from toes and fingers. This stops them from bursting into flames, but leaves them exposed to general attacks.
* Make fat as a material a little less volatile by altering its melting and ignition points.
* Make fat as a tissue have less of a blood supply. Not enough granularity here -- setting it to 1 makes fat melting easy to shrug off, and setting it to 2 makes little difference. (The default is 3.)
* Make toes, fingers, and facial features [INTERNAL], making them not directly targetable at all. I didn't like the results of this, though I don't remember why.
You might also want to tweak the healing rates of certain tissue types while you're working on exposed body parts. Nail and cartilage have no healing rates, making any injuries to them permanent and, more importantly, permanently open to infection.