multiprocessing was intended as a replacement for threading for platforms like CPython and PyPy that have a GIL. However, it might make sense for Jython to support. Such support would require mmap #1854873 and shared semaphores (via JFFI?).
Note that multiprocessing doesn't support cluster computing per se, and so it might be more worthwhile at looking how to better take advantage of Java support there, namely Apache Mesos and Apache Hadoop Yarn.