There is a modified pencil grip that I like to use with a MAG solitaire or Minimag that has a wrist strap on it, usually not the factory strap but a better one off an old cell phone or the like.
With the strap around your wrist, the light is in the pencil grip position, but you practically don't have to hold it with your thumb at all if you have the correct length strap, just use your pointer finger to keep it pointed where you wat by placing the tip between your pointer and second finger. It is very comfortable and works good in my left hand leaving my right hand free to work with other tools or keys.
I cringe when I think about a metal light between my teeth. It's psychological or a phobia about breaking my beautiful teeth. So, I only mouth hold a minimag with my teeth clinched, the light is in the side of my mouth mostly held in place by my cheek.
You asked about holding positions for lights mostly while they are in use, but what about while carrying them? I like a MAG 2C in my back blue jeans pocket. Or a minimag in a side leg pocket if I am wearing carpenter pants, which are about 75% of my jeans.