Yes Fenric's ERC will do exactly what you describe for the set driven key.  In fact I remember one of Faba's earlier test animations that was coincidentally enough an animation made of a M4 hand with sliders which would allow each of the fingers and thumb to grasp/bend/curl straighten, move away from each other, clench, etc individually all based on a few slider objects she had set up.

 

For what you describe as a proxy rig, I'm sorry but I have no idea.  I know you can take existing bvh animations and put them on any skeleton, but I have very little experience with animation and I don't know if there are problems inherent in putting an animation  which was designed for a more complex skeleton onto a simpler skeleton (I suspect you'd be able to do it, but it would look a little weird and you'd get people walking above the ground or through it, etc).  I bet sci-fi funk could answer this in a second though, he does tons of excellent animation and would know.  There are lots of much more experienced carrara users than me who do animation stuff all the time, so hopefully one of them will see your question and answer better than I can.