FYI, check out the maven "assembly" plugin (assembly:assembly). It's a standards plugin that's designed to do essentially what you're doing (package together classes, dependencies and such into some format and zip/tar it up).
Note that it requires yet another XML file for configuration, so it won't win any verbosity competitions, and it's still not as flexible as Groovy, but if you were stuck in the Maven world, I think it would do what you want.