I however cannot upgrade to 5.8 or 5.7 right now, and Sanne Grinovero told me such things will not be backported.

Right... Well, be warned that we made some heavy changes in the internals in 5.8 already, in order to be able to support multiple Elasticsearch versions (2.x and 5.x). As a result we introduced our own high-level abstraction over the REST client. Thus it may not be straightforward to port your work to 5.8.

Please keep us updated though, so we know whether to work on this on our own or to wait for you PR! And of course, feel free to ask for advice should you need it.