Andreas Esau shares a Blender Add-on that is designed to make constraining shapeKey drivers to bones an easy process. The Shapekey Driver Constraint allows you to connect the shapeKey driver to driver in the scene.
This addon makes constraining shapekey driver to bones super easy.
Simply install the Haddon through the user preferences and if you have shapeKeys, you can select the mesh object, then the bone while in pose mode. The Shapekey Driver Constraint works as an operator allowing you to set all the values applying them so the shape can be driven by the bone.
Check out and download the Shapekey Driver Constraint Addon for Blender here – if you do a lot of character rigging, this will be a good fit.