If you want to bridge between Web Services transactions and JTA/JTS transactions, then what you outline needs to be done. That's what we term "end-to-end transactions".
Ideally you'd like this to happen transparently, so you don't have to worry about spanning transactions from WS-to-JTS or JTS-to-WS: it just happens (tm). We had this in place back when this was the ArjunaTS (or a poc), but it hasn't had the priority to move it/complete it within JBossTS just yet (http://jira.jboss.com/jira/browse/JBTM-44). The hooks are already pretty much in place for this though, and JSR 156 defines the standard way to do this, or will once it's complete!
Feel free to vote for the JIRA task. I suspect this will happen sooner rather than later.
Thank you Mark for clarifying this.