I recall reading someone's experiments with beer fermentation measurements about a year ago. He had placed a pressure sensor in the airlock in order to count bubbles. If I remember correctly, he concluded that the pressure difference was so low that it was difficult to make trustworthy measurements.
As a fellow brewer, I'm actually wondering what you want to measure. Once the airlock begins bubbling, the pressure is going to remain mostly constant, so it won't tell you anything except that fermentation has begun. If you want to measure the fermentation "heartbeat," a bubble counter seems like a better choice.