Rigify: should I add bones to the metarig or the generated rig?

I’m using a combination of mocap on a character’s face, with a rigify rig to control the rest of the body. At the moment, I still have 2 separate rigs, using parenting constraints with the rigify rig on the head, rather than actual parenting. But this has become too unwieldy, and I’ve decided to join the two rigs together.

So the question is: is it better to join my mocap rig to the generated rigify rig, or to the metarig, then generate a new rig?

One complication is that the bones in the mocap rig have already been animated, so I wonder if both the new bones and their f-curves would be produced correctly in the generated rig.

For now, I’m going to make a copy of the mocap rig and join it to the generated rig. At least that way if everything goes south, I can get back easily to the current (albeit unwieldy) situation.