Thanks for all the suggestions (and the laughs!). Sorry about the late response. I wasn't expecting all this great feedback.
Hevans comments are spot on. I'm looking for a way to monitor and record grinding activity. Bruxism is a surprisingly difficult habit to break. As others have said, guards only protect your teeth. The constant jaw activity still causes pain and other unpleasant side effects. Without some way to record jaw activity, it's difficult to gauge whether certain treatments are effective. There is often a delay between a bad night of grinding and the effects.
Hevans idea of using a pressure sensor sounds promising. I'm picturing a small silicone tube that could be worn in a loop around the back of the neck. Mr. Ed style, like a horses bit. Biting on it changes the air pressure in the tube, which changes the output voltage of the sensor.
I've also been experimenting with EMG circuits. With a few electrodes taped to my jaw, I've been able to measure muscle activity. This approach is less intrusive than having something in your mouth, but it comes with a different set of challenges. Mainly, the signal produced by jaw muscles is very small, which makes it difficult to isolate.