I haven't tried using the unixODBC driver myself, but since the XA specification was written around 'C', the dynamic registration part of it isn't usually well supported by other types of interface, h ...
Did you ever get this working? If not it might be the unixODBC driver for DB2 is not registering with MQ. You could try doing static static registration, build yourself db2swits rather than db2swit, a ...