I do get quite many questions about this, so I pushed this up in my todo list. Can't say when it will be done, but if you'd like to get started with a custom solution, this is how I'd do it..
Make a mapping of the avatar's hands to the SteamVR hands. That would be just defining the forward and up axes of the finger bones:
public Vector3 avatarFingerForwardAxis;
public Vector3 avatarFingerUpAxis;
public Vector3 targetFingerForwardAxis;
public Vector3 targetFingerUpAxis;
Those are the local axes of the finger bones that point towards the next finger bone (forward) and outwards, away from the palm (up).
Then match the rotations with RootMotion.QuaTools.MatchRotation():
avatarFinger.rotation = RootMotion.QuaTools.MatchRotation(targetFinger.rotation, targetFingerForwardAxis, targetFingerUpAxis, avatarFingerForwardAxis, avatarFingerUpAxis);
That finger code must run after VRIK.