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.
Cheers,