I first tried out just creating curves for controls and orient constraining them to the individual joints, but when I did this and Froze the Transformations, the rotate tool on the control didn't line up with the way the joints were rotating, and that would have been awful to work with. If any of that made sense, haha.
So what I've done is just set up more controls with custom attributes. I have grown far too fond of set driven keys! I've given these a bigger range (-50 to 50) to make it easier to get precise adjustments.
The wee diamondy star things are the joint controls, I've coloured them to match the first controls.
So now, you can curl up the fingers using the initial controls, and then adjust them using these. Or you can fire straight in with these, depends what you're after really.
(1)- Hand with Pinky Control 01 attribute "Curl" at 5 and (2) with 01 still at 5 and Pinky Control 02 attribute "Rotate Joint 03" at 15.