What you are saying (I think) is that Sun's client-jar file (as built by Sun's "deploytool") is the source of my client-side ENC-like mappings. The J2EE server isn't involved at that stage. It doesn't (and shouldn't) export the ENC(s) of its beans at run-time. so, the server is not involved in mapping client code java:comp/env references to the server resources. The client-side support has to do all this.
Yup, I had not considered this way of doing things, but I think it does work this way, in keeping with the J2EE notion of mappings created at the time of deployment. Deployment mappings are not to be changed at runtime. Unfortunately. This means that in order to change the mapping of client references so that I can put in a subsititute EJB (on the fly) without changing the compiled Java code, then the mappings at each and every client must change (on the fly). Ugh.
Oh well. Someone will fix this someday. So, has anyone succeeded in getting a Sun client-jar and its "runclient" script to run against JBoss-deployed EJBs? Not that this solves anything.