Green Threads JVM's, allow for migration because each Java thread is a separate process. Threads other than Java green thread JVM's cannot be migrated by Linux, so openMosix cannot migrate programs that use them.
If you have the source so your Java application you might be able to compile the application native. In this case you might be able to migrate your applications to another node.
Gian Paolo Ghilardi wrote a paper titled Consideration on OpenMosix it deals amongst other topics with Java an dopenMosix. http://www.democritos.it/events/openMosix/papers/crema.pdf