Forgot to say I'm using 2.4.4/jetty 3.1.3 bundle and jetty configuration. The MBean example is in a jar in \lib and/or \lib\ext. The example MBean is found, but the superclass isn't.
Okay, I got it to work finally, by setting JBOSS_CLASSPATH=\lib\ext.
Could've sworn I'd done this before; must have bean too much coffee.
Sheesh I hate the Java classpath misery.