The programmers wanted to use bones instead of morphs to animate the facial expressions. Bones use less memory in-game but they take much more time to animate, so my goal was to export bone animations while keeping morph's simplicity. I came up with this:

The white lines are segments of one spline, which has a morph modifier that our animators actually work with. The spline is not imported in-game, its only an interface for the animators. The green dummies are our bones and they are linked to the segments of the spline by scripted controllers:

The animators didnt have much to complain about since I kept the interface they always worked with. It also made it easier for them to tweak the shapes without involving modelers.

I also added these buttons to the Editable Spline for the modelers:

"Create Morph Targets" creates an object of each morph target and link them to the modifier. "Update Morphs" is used when adapting the rig to a new face. It applies the modifications to the morph targets.