I have never tried it myself. But how about RandomRobin? Will that meet your need?
Interesting point you mention. I guess this is normal since EJBs rely on the dynamic proxy to decide to which node the request from the client application should be redirected. It actually contains a list of all nodes and this list is being iterated when you use round robin.
Activate load-balancing between different clients should be nice but I don't think you can actually do something about this behaviour. In other words, I don't think it's possible to control this... unless you start using hardware loadbalancers instead of software loadbalancers, but I'm not 100% sure about it :-S
Not a real response to your question, but perhaps it might be usefull for you...