I only have one thing to say ... SWEEEEEET!!!!
Been waiting to "script Eclipse" forever.
Couple things:
I've had mixed success is mixing Java and scripting languages. Eventually things devolve into "writing Java in xxx language", which isn't really a big help. I've also had specific problems with Rhino not being able to handle some particular Java constructs. Can't remember exactly, something to do with some classes in Java that were difficult/impossible to subclass correctly in JS, but you needed to per the API contract - had to resort to writing Java "covers" that I could then subclass in JS, or some such. I'm glad to see a concerted effort made here though.
The other thing of interest to me is trying to inject some modularity into the picture. Best effort thus far is some of the work proposed on the serverJS stuff -
https://wiki.mozilla.org/ServerJS . I think the current proposal that is favored is the poorly named "SecurableModules" proposal -
https://wiki.mozilla.org/ServerJS/Modules/SecurableModules ; this happens to be the approach I like best as well. Not sure how this would fit into the Eclipse work, might well not at all; I suspect you may be loading the _javascript_ code in separate contexts anyway.