You could add feature request but I don't think I will work on it in the near future (maybe someone else could contribute it).
The only left join fetching JBossCMP supports is by using left-join read-ahead. See wiki.
Why would this not be high priority? Turning on the global flag in our JDO provider almost exactly doubles the speed, I think the same would happen in JBoss. It seems like such a huge gain for so little effort, why would you not put this in?
Granted, we could run through the whole app to turn on left joins all over the place, but that would take serious effort, especially as left join syntax isn't supported by XDoclet. Our project is huge, and it's hard to sell people on CMP over JDO when the CMP system requires massive adjustment to turn on what should be (and is in JDO) a global pref.