I've made an alpha release of Mahlee™ available for download after posting a note about it on Lambda the Ultimate.
Neither messages nor threads are garbage collected yet, but the threading itself has been rock solid with everything I've thrown at it. It's proved to be a very useful systems administration scripting tool and it is now used extensively in our build process for running tests.
As a final note, from what I understand of Rhino it shouldn't be too complex to copy the API and approach. If you're interested in trying to do it get in touch.