I\'m looking to run some un-verified scripts (written in a yet-to-be-determined language, but needs to be Java-based, so JRuby, Groovy, Jython, BeanShell, etc are all candidates
In Groovy, you can do exactly what you mentioned. Actually very easy. You can easily limit permissions of untrusted scripts running in a trusted environment, allow usage of your own api, which in turn can do untrusted things.