Currently, you can inject beans that may not be ready yet into the extension's call back methods. As an example, I can inject something application scoped like this in to an extension, but it should really be throwing a definition exception (or similar):

public void handleABD(@Observes AfterBeanDiscovery abd, MyApplicationScopedBean masb) {

}

Pete had noted that really the only safe thing to inject, other than the observed call back, is the bean manager.