The code is not available publicly but I am considering exposing my ES client part (higher-level abstraction over the REST client or Transport client) and mass indexer.

I will probably send a PR indeed. Right now I am struggling to build upon HS functionality somehow but the best way would be to alter the internals. I however cannot upgrade to 5.8 or 5.7 right now, and Sanne Grinovero told me such things will not be backported.