Hello,
I was able to do this some time ago with VisualVM, but I used the JMXMP connector (used by JPPF) instead of the RMI one:
1. I had to put the JMXMP library "jmxremote_optional.jar" in "VISUALVM_HOME/platform/lib" (you can add any other library you need)
2. I used the JMX URL "service:jmx:jmxmp://server_address:server_port"
Can you try that and let us know if it works for you?
-Laurent