Hi Daniel,
Additionally to what Nick said, if you or someone else from the Freemarker community take over this code, I'd recommend trying to extract a language server from it and make this language server one of the Freemarker deliveries. There are already several good resources explaining how to do that with LSP4J.
Then integrating this language server into several IDEs would become almost trivial to implement and way more sustainable to maintain.
On a side note, I don't know whether additional support for Freemarker or migration to a language server is the kind of things you can try to buy from Red Hat. Bob (as CC) may be a good entry-point if you want to evaluate this possibility.