If I look for Spine and Solar2D this thread is always the top entry on google.
So as an update, I made a plugin that runs Spine 4.2 using the C++ Runtime. You can see the documentation here: Welcome to the Solar2D’s Spine Documentation! — Spine plugin for Solar2D documentation.
Or follow the plugin thread here: New native Spine 4.2 plugin - #13 by depilz.
At the moment of writing this the plugin is very stable, but still needs a few more tweaks which are coming soon.