Following Marks excellent advice, and after finally getting a prototype implemented I discovered that this very issue is already resolved for us by JCA.
You can see the recommended approach in: https://svn.jboss.org/repos/labs/labs/jbosstm/branches/JBOSSTS_4_15_0_Final/atsintegration/tests/classes/com/arjuna/ats/jta/distributed/server/impl/ServerImpl.java -r 37580
You can see the implementations of commit and rollback that do XAException acceptable mappings at: https://svn.jboss.org/repos/labs/labs/jbosstm/branches/JBOSSTS_4_15_0_Final/ArjunaJTA/jta/classes/com/arjuna/ats/internal/jta/transaction/arjunacore/jca/XATerminatorImple.java -r 37579
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