Several years ago I came across the same issue. I had a character who has to be keyframe-animated, but also mocap-animated sometimes. And even, both in the same shot! I mean, for instance I had a shot where the character was animated by hand at the beginning, then I had to insert mocap animation for 10 seconds, then back to hand.
The solution I used for this to work and to be able to switch between each type of animation seamlessly was the following:
- Using one Rig to deform the mesh and animate it by hand (FK, IK...)
- Constraining some of this Rig's bones to the mocap Rig (the one which was created when I imported the Mocap data) with child Of bone constraints.
Then I just had to set the last hand-animated pose according to the first mocap pose, and animated the Influence of all "child of" constraints to switch between keyframe anim and mocap anim. Here is an example of setup, in use:
Of course the constraints had to be created with a small python script, once I found the good list of bones that had to be constrained (like head, neck, shoulders, FK arms, torso, FK legs, etc, but not the root bone). So finally I made it once, created the script, and then all similar cases were just at one button to go.
And, since all my mocap data was created with the same software (motion builder if I well remember), the imported Rig was everytime the same (same bones names, same structure), so my script worked like a charm. And also, when you need multiple mocaps in the same scene, each time you import a new mocap, no need to re-run the script, you just have to change the animation data of the first mocap-rig, which is already "binded" to your character's rig.
I hope my experience will inspire you! Best regards