Animating a shape key that has been hooked to a driver I'm using VMComix's excellent fully rigged Porl character (https://www.blendswap.com/blends/view/14445) to experiment with character animation and have found all the sliders in the N menu that allow for animating common movements like making an open hand into a clench, etc.

However the character also has facial expressions - I'm trying to get him to go from basic facial expression in frame 1 to 'Jaw Drop' in frame 20.

I only seem to be able to activate the Jaw Drop from the Graph editor by switching type to 'Scripted Expression' and Expr to 1. However there doesn't seem to be any way to keyframe this.

I'm a Blender amateur and I get a feeling I'm overlooking something incredibly obvious .... any pointers would be greatly appreciated.
Custom properties on face bone.

Would leave the drivers as, is and animate the custom property that drives the jaw.  In this case its the "jaw_down" prop on the "face" pose bone.  Can see this from  the driver variables in your screen shot, just below your red circle.

Select the face bone in pose mode and the custom properties are shown on bone properties panel (as in image below) or as stated in your q, in the 3d view properties panel N