Don't know if this is of any use to you but we have a working mssql-xa-ds.xml.
You don't say what problems ~you're~ experiencing but for us it required the installation of additional stored procedures and a DLL file on the SQL Server to get it working. Both of these came with the latest drivers from MS.
My guess it the PostgreSQL server may also require the same if you haven't already done so.
Would be interested to know once you do get it working whether you notice a drop in performance. We've notice transactions times taking 3-4 times as long.
It seems thet XA drivers doesn't exists for PostgreSQL.
Read Aaron Mulder's comment in sources of jdbc driver in web cvs (thanks to Jon Barnett).