I’ve written a very simple finger component based on chain_01 that adds a curl attribute rotating all digits in the -z axis. The attribute name is chosen uniquely so that multiple fingers can be attached to the same UI control.
Would there be interest in my sharing this upstream?
Is this attribute adding to the current finger control rotation value? Or is it overwriting it? The experience I had with animators in the past was that they did prefer to handle their curves individually, so a ‘Curl’ attribute did not really add value to a rig. But, of course, not all animators think the same.
Just my two cents 
It adds an extra SDK group above each control with the z rotation driven by the curl attribute.
If you are going to contribute it as a component, may I suggest using remapValue nodes rather than driven keys?
remapValue nodes evaluate a tiny bit faster. They will also never show up in the animator’s graph editor under any circumstances. (Who knows? Someone might select an entire hierarchy. There seems to be a lot of beginners who use mGear as well.)
I also had a weird production issue once where all the driven keys in a referenced rig ended up having null information. The keyframe nodes existed, but no keys! So everything collapsed to zero. The rig file appeared fine. It’s a super rare bug, but remapValue would definitely be more stable and tamper-proof.
I meant to say “driven groups” instead of “SDK groups” - the implementation just uses multDoubleLinear to scale the rotation. But those are very good tips and I will make an effort to avoid using actual SDKs inside rig components.