No, unfortunately not. In my opinion it does not make sense to setup a JTA ressource in GF and then specify the target database in the persistence.xml file. In the connection pool there are facilities to apply properties of any kind, and most of them have the same functionality as in a persistence.xml when used in a standalone application (like user, passwd, host, etc.). And I can tell you that I got it once running, putting a jar with the overriding class into the GF/lib directory. My single point is that this one is not loading anymore with the more recent versions.
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