Sounds interesting. The article from Martin Fowler (http://martinfowler.com/articles/languageWorkbench.html) talks about language workbench's like this in 2005 and this seems to be the first available mainstream implementation of one.

Intentional Software (http://www.intentsoft.com/) also promise a package like this, but haven't delivered anything publically as far as I'm aware. There's also JetBrains MPS, but not much seems to be said about that any more.

All of this seems to be a kludge though. Java's weak at allowing new syntax (no operator overloading, no higher-order functions, no dynamic dispatch) so you need verbose code to implement your own language. With a language more targeted at this, such as Lisp, would you need all this?