Hello,
I was looking at the JBossTS source code and I found a JAX-RPC context handler:
com.arjuna.mw.wst.client.JaxRPCHeaderContextProcessor
There is no official implementation yet, but I have a prototype I used during the BOF at JavaOne. Have a poke around in http://anonsvn.jboss.org/repos/labs/labs/jbosstm/workspace/jhalliday/txbridge/ and you should be able to find it.