In this tutorial [www.youtube.com], Adam (the instructor) demonstrates a method for bringing in BVH mocap data and retargeting it to an existing Houdini character. I imagine that this method could be used to apply mocap data to a Houdini character and then that motion could be recorded to a clip in the Pose Library. The Pose Library would then provide the means of using both the mocap and additional keyframe animation on top of it (i.e., hybrid animation). Wouldn't this be a feasible approach? Is there a better method?

I haven't seen that video before. If you're using the Pose Library, then you can just change the “Character” from one rig to another to retarget. As for doing additional keyframing on top, I think you would need to use Animation Layers, which are independent of the Pose Library.

I've never tried the workflow that you want so I'm just guessing as much as you are. I think you really just need to give it a try using say one of the default character rigs that come with Houdini and see what works, what doesn't, and then ask questions on how to solve it.