I think Unity provides many ways to navigate the object hierarchy.
For example, you can get the active HandModel objects from a HandController with GetAllGraphicsHands() or GetAllPhysicsHands(). You can get then get FingerModel objects from the fingers[] array of HandModel. And you can get the transforms for the bones of the fingers from FingerModel.bones[].
Another way you can then get the transform for a particular part of the hand is with Transfom.Find(), which can take a path-like string to the name, such as: "HandContainer/Bip01 R Hand/Bip01 R Finger1/Bip01/R Finger11". When you have the transform, you can get the game object.
Yet another way would be to tag or add marker scripts to the hand prefabs for those parts that you are interested in.