Did some reading on the Box2D site and discovered that a good way to get a pressure plate / push button effect is to use prismatic joints. I added a quick prismatic joint (via a custom push button class) to my test arena and it worked great -- just needs some tweaking to prevent bouncing, etc. The next step will be to tie the opening / closing of a door to the button being depressed.
With all of the experimentation and exploring I've been doing lately, I've thought about maintaining all my finds / tips / tricks in a cookbook format to act as a companion to the Box2D manual. Sure, there's the demos that are included, but it would be nice to have some written text so you don't have to work at deciphering how to implement a particular activity.