If the default way is used to connect the server it is possible to get a reference for a UserTransaction with
EJBClient.getUserTransaction("node-name")
If a scoped-context is used the default environment might be not set or different and the correct UTx object can not be received.

A simple way to use somthing similar to former releases as
InitialContext.lookup("UserTransaction")
might be a good way as there are no JBoss specific classes necessary