Ok. "Groovy shell" means something specific to me that has nothing to do with what you are asking... so I'm glad I didn't respond before.
Groovy can call Java code. So you can use any Java class you want from the groovy scripts that Mythruna uses. I think you just have to package up the Java .classes in a .jar that you put in a lib subdirectory or something under where the scripts run. You might just be able to put the .jar in the same directory as the scripts, I can't remember.