Ah, you already have global state:
globalAutoUpdate
I just ended up adding two lines to my PIXI Spine runtime to do what I needed - much simpler than your recommended approach.
Spine.globalTimeScale = 1.0;
and, in the autoUpdateTransform method, divide the timeDelta by the globalTimeScale
timeDelta /= Spine.globalTimeScale;
I think adding this (tiny, two line) feature would be beneficial to many developers, instead of having to implement synchronization themselves.