LOL! This subject has crossed my mind as well. Not exactly a German Shepherd but I've thought shoving my fist and half my arm down it's throat and choking the son of a "bitch" to death. Sure you'll feel some bite pains as your clinched fist gets near the mouth but if you can get in there his biting ability will be greatly diminished once you get your fist and arm down.
I read somewhere that if a pit bull has you in a vice grip - you can get it to release by grabbing it's a-hole. Of course... if this didn't work and the dog proceeded to kill you, it would be a really f'd up way to die.
I would also be trying to snap limbs in a fight like this. you bring a knee down on a shepards leg just right and I gotta believe it snaps.