driven shape keys?

In 2.5, how do I make a driven key for a bone rotation that gets its rotation through constraints? I want to use a shape key to fix an elbow deformation, and the rig supports ik/fk, so I want to have the shake key driven by the deforming bone rather than any of the controllers, but this bone always shows the rotation values as 0.

Right now I’m trying to get the value from pose.bones[“def-arm-1.R”].rotation_euler[1] in the driven channel, and changing this value will drive the shape, but the local position that it’s in relative to the next bone up on the chain (local rotation?) seems to be ignored since the rotation is inherited through the constraint.

Sorry if this is the wrong forum, all of the 2.5 questions seem to be posted here.