In little quickie test scripts, if editor-scrubbing uses a normalized time of 0 only makes the first frame show up and seems not to continue. However, when a similar script is applied to a Unity created anim and mecanim, it works as expected... (I can move a cube around on some quick anims managed by a simple mecanim and only when normalizedTime progressed). This script does not work with Spine's skeletonMecanim though.
Executes in edit mode...

Can confirm the correct information and clips are reached in the mecanim graph.
To make things more interesting, and hopefully I'm wrong about this, seems (in the editor mode with respects to mecanim) the default mecanim values get reset to their defaults every editor frame. So guessing that's why we'd need to tell it what state and what time it needs every update. For a normal cube animated in editor time, it only animates when the normalizedTime parameter continues to increase.
What's missing for the mecanimSkeleton to update through a whole animation at editor time? This logic would then be planted in the timeline mixer behavior.