Oh, then I had to modify the physics engine to actually send JOLT events out. It already sends out CONTACT and ~CONTACT messages to let you know when you start and stop touching something, but the JOLT message is different in that it indicates when your object has actually hit something, and how much energy was involved in the collision (so you can distinguish a glancing blow that a bomb might bounce off from a direct impact that should make it blow up). I got this mostly working, but wasn't able to find a place I could easily detect situations where you were hit hard, but didn't move (e.g. you're already jammed up against a wall). Still, it's enough to get me started.
At this point, I had my projectiles hitting things and calling a stub "Explode" method which just removes them from the scene, and I started thinking about how I was going to depict explosions. Given I don't even have the start of a particle system yet, I quickly decided I wasn't about to get any explosions working before my train arrived, so I started mucking about with other ideas ... like how much I enjoyed the geometry cannon from a few journal entries back.
Here's a moovie of one of the prototypes. I don't know if I'll end up using it as it has some obvious game balancing issues, but it was fun to play about with and took a grand total of 3 lines of code to implement.