>>1026635 you mean you want some kind of adjustment bone? it's the same as normal driven shapekeys then, you just have a non-deforming bone parented to whichever part of the arm which will control your drivers instead of the joint bones themselves. watch a basic driver tutorial as >>1026641 said
A while since I've done this. You'd want to use the eulerangles of posebones in your drivers. Blender has some sort of autocomplete for selecting an armature and it's bones for driver source but that doesn't work. It has to be posebones. Also found this: https://extensions.blender.org/add-ons/easy-driver/ Probably easier to set up. They show the exact use case you mention.
theres def gonna be a time to use shapekeys, but i cant be fucked to learn drivers when the sound of the impending nodification of blender on the horizon.
All we need now is a node for shapekey fields themselves.