Is it possible to use an active rigid body with keyframes?

My idea is to key a little bit of reaction animation between frames 65 and 75, then let the solver take over for the rest of the animation.

Yes. Animate the object first, then make it a rigid body and keyframe the Active attribute of the rigidBody node. If you make it a rigidBody first and animate it later, you can also do it, but it’s much more complicated, due to the insertion of a pairBlend node. Better not do it that way if you’re new to this.

Here’s what I’m doing.

I have a brick wall, with separate brick meshes.  I selected the bricks I want to simulate.  At frame 99 I make them active Rigid Bodies.  At frame 100 I turn the state on (on each of them).  I have a skinned character which I constrained a set of spheres to.  I turned these into passive rigid bodies.  I do the same thing.  At frame 99 I key the solver to inactive, and 100 to active.

But nothing happens :(

There may be a couple of reasons. Sometimes when the rigid bodies are packed very tightly, you may not see the impact of a colliding object. I tend to scale them a bit so they have some space to move. Th other thing is timing. If the passive rigid body becomes active at the same frame at which impact occurs, there may not be enought time. You might want to give it an extra frame.