Thanks @Mitch @Pharan and @wrongtarget!

I'll not convert to mecanim in my current project but will try it out for the next one.

@wrongtarget: Could the separation in the parts have something to do with the 0.1m separation along the z-axis that bones are arranged on (according to draw order in the Spine editor)? Try changing SkeletonComponent.zSpacing to 0.0f and see what happens.

I just started this thread where I ask why zSpacing is set to 0.1f right now: viewtopic.php?f=7&t=2121
I'm curious to see if setting it to 0.0f breaks anything with your shader.

Disclaimer: I don't know the first thing about writing shaders.