Looks like with JRuby and in particular now that it has JRuby Embed (old name Red Bridge), it should be easy enough to do a similar setup I did for Groovy and passing shared data of OpenRefine objects and fields to JRuby, according to their docs.
I'll work on that next week! Stay tuned for the next scripting language support!
The idea for adding more scripting languages, is so that we can try to cultivate even more users of OpenRefine that might be scientists or data architects that might know a particular scripting language already. All the while we can continue to try to make GREL even better and easier to use in the future (like new functions, better error reporting, linting, etc.)