Pose Sliders?

Is there a way to make a slider (like an IK/FK switch) control the influence of a pose from the Pose Library?

I’ve been doing facial animation with a full facial rig and a Pose Library of visemes, and was thinking about switching to shapekeys for the speed gained when it comes to lipsynch, but am apprehensive about either giving up the freedom of doing expressions by directly manipulating the face, or having to manage a complex hybrid of both methods.

However, upon rewatching Christopher Crouzet’s reel, I notice that the facial rig has a pretty elegant solution: pose sliders, which determine the influence of poses which directly manipulate the control bones. A pure rig solution for both methods!

EDIT:
Aha, I think I’m on to something: Action constraints! Testing now…