There is a new release 67of C3 with this comment, hopefully this will help badlogic get going (and to request for features they need) (I also posted this to the Spine github, feel free to pile on!):
"Additionally we've started working on adding support in the SDK for drawing plugins (i.e. plugins that appear in the Layout View). This involves quite a large API surface, so it will probably take a few releases to mature. We'll be updating the documentation soon to cover the new features. We're interested in developer feedback to make sure we add all the features necessary for third-party drawing plugins to work."